Thread: Hartley 18' half cabin - help re outboard hp

Originally Posted by andrew allan

...........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.

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.

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."..........!!

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.

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

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.

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.

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

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"

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.

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.

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.

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.

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?!!