Hartley 18' half cabin - help re outboard hp

[andrew allan]

I've just been lucky enough to pick up an old Hartley 18' half-cabin in good condition, and was wondering whether someone can help me with outboard motor size.

I'm told the boat had a 115hp engine on it before, which suggests it is a planing hull (or owned by an idiot!), however I'm not in a position to go buying large outboards at the moment, and was wondering whether it would go (usefully) with a 5hp engine. I don't want to launch it to find I'm getting sucked out Port Phillip Heads as I can't go against the tide!

I'm a little unclear about issues of displacement vs planing hulls - does a planing hull work like a displacement hull at speeds below planing speed?

Is there a useable equation in terms of working out likely speed of a displacement hull in terms of weight on board

[Seafarer]

...........was wondering whether it would go (usefully) with a 5hp engine. I don't want to launch it to find I'm getting sucked out Port Phillip Heads as I can't go against the tide!

I'm a little unclear about issues of displacement vs planing hulls - does a planing hull work like a displacement hull at speeds below planing speed?

Is there a useable equation in terms of working out likely speed of a displacement hull in terms of weight on board

Adnrew,

5hp is probably bare minimum for a boat this size. If dead calm conditions it would move it along OK, but tide and wind would work against you any normal day. If you can get a high thrust propeller you might get away with it, otherwise 9.9 or 15 hp would be closer to the mark I reckon.

Can't answer your question about behaviour of a planing hull at displacement speed sorry.

Cheers, Cameron.

[Paulmaximus]

One option for the Hartley 18 was a 45hp inboard motor.

My suggestion would be a 6-8hp for safe displacement motoring and a 50 hp for a planning hull, but only the latter if the transom area is strengthened to cope with the power and weight.

A planning hull works like a displacement hull at low speeds. However it will be flatter resulting in more drag resistence compared with a hull designed for displacement speeds. Thus it will require more power to reach a given speed.

The traditional calculation was the top speed of a displacement hull is approximately in knots the square root of the waterline length in feet. A very efficient hull will reach 1.3 times that typical speed. The Yachting Australia minimum safety requirement is the power should be such that it will result in a speed of 1.8 square root of waterline length in meters against a twelve knot headwind. In practice on a Hartley 18 a 4hp is a minimum, but for motor only use a 6-8hp is better. A good rule of thumb is 1 hp per 125kg dispacement weight for small yachts. To break out of the displacement hull wave pattern requires substantial power, thus anything above 8hp on a Hartley 18 would be a waste until you have a 50hp to jump up onto a plane.

[andrew allan]

Thanks - that's a fantastic response.

I'll get out my tape measure and calculator, and get my head around the sums.

I already have a 5hp - I was trying to work out whether it was a complete waste of time putting it on the boat this weekend and seeing what happened, but it looks as though it won't be. I'll just be careful with the tides and wind, and see what happens......................."News Flash - Mainland Ozzie ends up in Tasmania after miscalculating boat power requirements."..........!!

[Paulmaximus]

Andrew,

At least you will not need a passport. As you will have paddles anyway, try for Queenscliff on the way out as there are some good restaurants. These days a mobile phone is often far better than the radio in an emergency, so you can ring ahead of time and make a table booking.

The Trailable Yacht Association of New Zealand issues a minimum outboard engine size for each class. For a Hartley 18 that is 5hp so you should have no trouble on the Bay.

[wavedancer]

Hi Andrew, I have a 10hp 4 stoke Yamaha bolted to the transom of my 32ft, 10tonne motorsailer (I seized the 50hp Perkins). Fitted with a highthrust prop and at half throttle in perfect conditions I can get 2.5 to 3 knots. I hate missing our club happy hour hence the drastic dissision to do this. A word of warning with an under powered boat things can go pear shape very quickly and I am sure the insurance guys would not be amused by my antics. I took WD from Pittwater to Gosford for new years eve and at times things got very "interesting".
I will post a picture from that night ( I hope I am not out of order in doing so).
Ian L

[PAR]

If this is the hull form I suspect it is, then a 5 HP will get it to about 7 MPH, a 10 HP will get it to the 11 MPH neighborhood, 15 HP to about 14 MPH and a 20 HP will push you to about 16, maybe 17 MPH. With these small, portable engines you'll get very good fuel usage (GPH).

If you want to go faster a 40 HP will reliably get you up into full plane mode and she'll scoot along in the low 20's MPH range. 50 HP will get maybe 25 - 26 MPH, 60 HP you'll just kiss 30 MPH in smooth water. If you double this HP you might get another 8 or 9 MPH out of her. I wouldn't try to force this hull much past 100 HP.

[Paulmaximus]

Unfortunately the response by Par is flawed and is against the fundamental principles of hydro-dynamics. The maximum displacement speed of a Hartley 18 half cabin cruiser is just under 4.5 knots which can be achieved with 4hp. Taking into account the length:displacement ratio, to break out above the speed of the hull wave system requires something in the order of 30-40hp depending on weight. The hull will assume a bow up position as more power is applied at 4.5 knots until the plane suddenly occurs, speed will increase rapidly then the bow will drop and power can be slightly reduced. To keep a Hartley 18 on the plane requires a minimum speed of some 10 knots which should be possibly on a throttle back power output of around 25 hp. The shape of the hull has an impact on the planning angle thus the amount of power required to met drag resistence. For the Hartley 18 the geometric power:speed curve is significant compared with a hull designed for fast cruising 90% on the plane. The hull has significant rocker curve, this is why the maximum design power for the inboard version is 50hp. Anything above that is sub-optimal.

In simple terms the power to speed curve is not smooth, it is bipolar. Thus anything between 6hp and 30hp is a waste of time. This is why recreational half cabin fishing boats have a 50hp outboard to plane out to their favourite fishing spot in quick time, but only a 6hp stand-by to displacement motor back home when the main unit fails or has a major propellor snag.

[andrew allan]

Bit of a Pandora's box opening up here.........

I really appreciate everyone's learned input, and think it might be best to sit back and watch the discussion reach a conclusion.

I attach 3 photos of the boat, in case this changes anything, as it is an old boat, most likely a Hartley (but possibly not), but due to age, it possibly has a different hull design to the current crop.

Andrew Allan

[Paulmaximus]

Thanks for the photographs. In my view she is not a motorised version of the Hartley 18 called the Fisherman 18. Neither is it one of the Hartley powerboat designs such as the Flareline 18 which is a planing sportboat/cabin runabout.

However, your boat looks similar to this latter design. With twin 40hp outboards the published speed is 35 knots, while with a 100hp inboard the speed is 30 knots. Your boat with that kind of power will have a similar performance.

The Fisherman 18 based on the 18' Trailer Sailer hull with a larger cabin is only for a 5 to 20hp outboard being a displacement hull. The larger engine is prefered for running on low throttle for extended periods at 4 knots. Most commercial operators however go for an inboard to give more working space and economy.

Back to the original question; your boat with a 5hp outboard will have a practical speed of 4 knots as with the Fisherman. She will cope with moderate conditions on Port Phillip Bay with no difficulty. Check your fuel houly consumption and carry 50% for safety. The main thing to watch is local tidal conditions in case the tidal stream is strong at certain times. In addition of course you will have checked the weather forecast! The Victorian Recreational Boating Safety Handbook issued free by Marine Safety is useful.

"There is nothing, absolutely nothing, half as much worth doing as simply messing about in boats"

[PAR]

Yea, you're right Paulmaximus, I don't know a thing about hydrodynamics, nor the hull form Andrew mentioned.

Andrew, if you have any common sense, do a reasonable search on this site (and others if you like) about me and of course your boat. The basics and generalities Mr. Maximus speaks of can be derived in the first chapters of most yacht design reference volumes, which suggests a possable working knowlage, but little else.

In short, my figures are impecably acurate within a few percent and of course the loading.

[andrew allan]

Hmmmmmmmm. Plenty to read between the lines, which is, of course the problem with the written conversation!

Being used to my little dinghy, I'm used to being a fair-weather boaty, and intend to stay as such with my 5hp, until such time as I have a spare $x000 for a "proper" motor.
I'll look carefully at the tide and weather conditions, as I always do, and even try to do some measurements of speed myself. Might even get it planing with my 5hp!!

At least it appears that there are 2 windows of hp vs speed, in which you get adequate performance, but above which further hp and dollars don't add that much.

It would appear that paulmaximus is a lucky enough Ozzie to be enjoying life in Bermuda at the moment (or someone who has enjoyed our local climes). We can but dream of such a life - particularly when it is 10 degrees, raining and windy in Melb, as it is today!

A.

[PAR]

Maybe I was a bit harsh, he does seem to have some understanding, but frankly driving a full plane mode hull form at displacement speeds is silly. The boat will wallow, wander around, be difficult to steer and maneuver and fuel use will be high (in comparison to full displacement mode hull forms). You certainly can find a good used 40 HP for the cost of a new 5 - 10 HP. In the current markets you can pretty much steal what you want. I just purchased a 2003 18' Proline CC with 2006 90 HP Honda, for $1,000. The guy was desperate and one of the axle bearings was frozen. I had to change out the bearing in the guy's driveway. He was clueless, saw the bearing mess and figured the thing was about to fall apart on him and was glad to see it go. I'll make a few grand on this one.

Small to mid size outboards are the most common types and when they're this plentiful, you can get deals. I got a 50 HP Mercury last year after it had sat in a local repair shop for how knows how long, with an outstanding bill on it. I paid the shop bill, plus a little and it runs great for half the price of a similar unit.

[Paulmaximus]

Hi Andrew,

I have the best of all possible worlds, living in St George Bemuda married to a fantastic blonde Ozzie and work internationally. Our son did one of his law degrees at Melbourne Uni, so I know the Bay and the city of four seasons in a single day. The family has been in shipping and shipbuilding in the West Indies for four hundred years so I have a passion for wooden boats owning a 1891 38' foot gaff cutter and a 1915 30' motor cruiser.

I have read a few books on yacht design starting with Chapman's 1768 publication, through Kemp in the 19th Century to Kazunori, and sometimes progressed through the first few chapters. However, I admit the latest yacht design software by people like Humphrey is a little more complex than my maritime law books, even the San Remo memorandum currently in the news over the Gaza Flotilla.

In one respect I agree with PAR's latest comment, there are occasional problems at the displacement/planning intersection. I have some amateur involvement in the design and operation of rescue craft where it is difficult to get a design balance in handling between flat out speed to the incident and lowspeed manoeuverability during the actual rescue. Costs of secondhand outboards are around US $5k for a 60hp in Australia, that is why with the present glut of secondhand cabin cruisers on the market in the USA they are being exported in bulk.

Anyway enjoy your boat on the Bay this holiday weekend, the temperature should be around 15 C. The 5hp will get you home, otherwise have a VB on me.

[andrew allan]

Went to VicRoads today to try to register my new boat (this Harley, or whatever it is). WHAT A SAGA.
The person I dealt with had to get a supervisor to help her. When I told them that it was going to be used with the same 5hp engine that I have on my other registered dinghy ( and having assumed that they would be able to provide me with the engine number from their records), I was told that they couldn’t.
She started mumbling something about privacy, but then stopped when I reminded her that it was my boat, and that I had just identified myself with my licence. The onus however, was apparently on me to provide them with information, not the other way, and there was no way she could give me the engine number from their records.
She then went on to say that even if she had been able to get those details, it was not possible to register one engine number to 2 boats at any one time, and that if I was going to register my current engine with the new boat, I would have to unregister the other boat first! AND, GET YOUR HEAD AROUND THIS, that if I didn’t want to unregister the old boat, the new boat didn’t need to be registered, as it didn’t have an engine on it yet!
Maiden voyage will therefore be in a boat that looks registered, but is going to be carrying the rego details/label of my old boat! How does one deal with that sort of lunacy?!!:no:

[PAR]

Andrew, that may seem like lunacy, but it's not uncommon. It's also very likely not true. By this I mean outboards under a specific size (usually 25 HP and smaller) are considered portable. In this, they can be carried from one boat to the next, at will and you don't have to transfer registrations or other paper work. What you will have to do is deal with someone that has a bit more authority then the last person you dealt with. In other words it's not unreasonable to use the same engine on two different dinks and they darn well know it, so talk with the supervisor next time.

You will not be at all pleased with the performance of your new found pride as a 5 HP putt putt. It will wallow around and handle like a pig, assuming that the smallest of waves doesn't slap you to a stop as each passes your bow. When more then a zephyr comes up and/or a contrary current, you're going to find out how grossly under motivated your boat is, possably not going anywhere, just holding your own (maybe even loosing ground) against a wind and/or current.

If you desire a putt putt, then 10 HP would be a minimum. You'd have some reserve so you can throttle back to conserve fuel (the 5 HP will be WOT all the time, which will kill it quickly) and you can prop her up (the 10 HP) to gain more efficiency (and ease the load on the engine), such as it will be, given the limitations in her shape (she really wants to get up and scoot).

I wouldn't recommend trying to "fake out" the authorities with registration numbers from different boats, etc. You may get away with it for a time, but if you get caught, I'll bet it's felony, so, consider your choices wisely.

[wavedancer]

Welcome to the registration nightmare Andrew. Wavedancer was layed up at Iluka NSW on transit from Harvey Bay Qld. I was up here in the mountains and I thought I would do the right thing and tranfer rego from Qld to NSW, the Qld mob didn't want to know about because the boat was in NSW. The NSW mob wanted me to take the boat to Coffs Habour so it could be measuered and a new hull number fitted even though it had the original nsw hull number. Short version is I paid $35 for the bloke who fitted the plate to say yea I put it there.:((

[andrew allan]

Dear Wavedancer,

Lucky you to get out of it so cheaply! I would have paid $50 not to have been so off with them! the

I suppose we just have to deal with the fact that when we go to such Govt organisations (inc passport's office!!!!!............don't go there!), we just have to realise that they don't care about the outcome - the outcome is our problem, and we shouldn't get irritated about anything.

I'll check my outboard engine number this weekend, and I'm sure that there'll be some transcription error next week when I try again to re-register my boat and engine................ where a "7" might look like a "1", for example, but it wasn't my transcribing that caused that error, was it?!! Every person with 2 boats must obviously have 2 engines, and as long as the engine numbers don't match, the computer can't pick it up

And, beyond this, who actually cares what the engine number on my Tohatsu 5hp is??? If you have a boat that has a rego label, and all the safety gear, it is unlikely that anyone is going to check further. It's only if you are beligerant, or difficult to them that they would even consider checking an engine number.

When observing the police launch going within 50m of the 12' tinny with 5 guys without life jackets a few months ago, I realised that you have to be pretty special to stand out!

PAR, I'm not expecting the 5hp to do anything special. The guy in the sea-kayak who paddles passed me this weekend, might stop me in my blocks with his back-wash, but hopefully there'll only be one guy who does so!. The 5hp is basically a "transition" engine, which will allow me to use my new boat in good weather (and the forecast is for this) until I can afford a "proper" engine

In due course (ie when I can afford a proper engine), I might ask more about the hp issue - my gut feelings at the moment are that an 80hp would be more than I need, and 50-60 possibly enough.

In Oz, there is still a significant $ difference as you go up the hp, so I'd still love some sort of equation that would allow me to plot out the hp vs speed curve based on boat length/width/weight/profile etc etc

Any further input would be appreciated.

A

[PAR]

Don't change a number on your engine's ID, change a letter. If you change the numbers, the computer may find that number registered to someone else's boat and think you've stolen an outboard. An "M" could be an "N", a "Q" looks and lot like an "O", a "B", could easily be mistaken for an "R", etc., this way you could apply the "transcription error" routine to the authorities if it comes up.

A 50 HP would be just fine, with plenty of reserve to punch through chop and contrary currents.

[Paulmaximus]

Victoria has complex boating regulations. Unfortunately there is no eqivalent of an International Driving licence so I had problems when visiting there for a world championship and wanted to take out a coach boat.

I agree with PAR just add a number to the end of the existing sequence. There is a nationwide log of stolen outboards so just changing the number is a marginal risk.

Regarding your request for a speed/hoursepower calculation. While you do not have a Hartley 18 cabin cruiser [Fisherman based on the Trailer Sailer hull] , your boat is very similar to the Hartley Flareline series which has a Vee shaped hull. Go to www.hartley-boats.com (http://www.hartley-boats.com) and check the Flairline 18 details which include speed and power up to 200hp, then decide what you are going to use her for. You could also look at the Hartley Vixen which is the Deep Vee series, so look at the Vixon 17 for a comparison with up to 220hp. One criteria, check for dry rot then your bank account.

A marginal problem is that different methods of calculating hp on outboards are used so the published output can vary by 10%, but that is another story!

[andrew allan]

Regarding putting my little 5hp on my new old boat, I'm trying to sort out transom lengths, and where the prop should sit under the boat . As it sits on my little dinghy, with a 43cm transom, the fount things on the side of the outboard shaft are at about the level of the bottom of the boat. The new/ old boat has a 53cm transom, which still leaves all the prop exposed below the boat, but the side fins are above the level of the boat bottom.
Basically, is it worth my while fluffing around making an extended/ false transom 8 cm lower on my new/old boat or will it work just as inefficiently (!) if I just bolt it on?

Particularly pleasing aspect of today was that I've towed my new/old boat 100km on it's trailer without any problem. We only have another 2km to the boat ramp. At $650 for boat and trailer, things are starting to look up!

A

[PAR]

The side fins you're referring to is (I'm assuming) the anti-ventilation plate. This is what appears to be a flat piece just above the prop. This plate should be at or as much as an 1 below the bottom of the boat.

Speed and general performance predictions are that hard, but you have to know what you've got. By this I mean the hull type, the weight and a few other "qualifiers", or the predictions are difficult to attempt.

You have a classic double wedge, warped bottom hull form. The transom shows about 7 degrees of deadrise. This and a rough guess at weight and you can get fairly close, but really, what's the difference between guessing 26 MPH and actually going 25 or 27? Your hull likely is about 700 pounds, plus 300 for engine, controls and fuel, you'll be operating this boat with 1,200 to 1,500 pounds of full up load. Warped bottom hulls need less HP comparatively then more modern constant deadrise designs, so the figures I posted about will be close enough.

[andrew allan]

Hmmmm.

As there's little wind this weekend and I know when slack tide is, I'll just give it burl the day after tomorrow and see what happens!

I really appreciate everyones input.

A

[andrew allan]

Well, we got out in the boat today with the 5hp.

Steering was aweful ( we went in a S shaped course!), but it otherwise went along ok. Best of all there were no leaks, we all had a ball, and we caught 2 whiting and a flathead!

Now to start saving for a proper outboard!

A

[malb]

Re registration issues, I believe that if you reg the big boat, you can operate the dingy as a tender to it on it's registration. Doesn't get around the problem of supplying the engine number of the outboard, but should get around the issue of using a common engine number with two boats. However I suspect that it could be hard to explain that the dingy is being used as a tender to a boat high and dry in the carport 20KM or more from water if that were the case.

[Hubcap]

Hi mate, i have a 24ft vixen hartley with a 25hp on her and she is fine. maybe put some sought of fin in front of the motor and bit of weight in the boat.sits quite comfortably on 6/7 knots.