Thread: stevenson project

Originally Posted by Boatmik

Howdy Dingo (again!)

BTW there is a bit of work involved in working it all out - so please don't let me do it if you change your mind! If something serious intervenes when you really think that you are going to do it - no probs - after all I love you like a brother.

But if you aren't going to do it then don't let me go ahead.

MIK
hehehehehehehehe

Gawd mate Im glad you laughed at the end of that!! had me worried for a bit you did
All this is only going to really change a) the performance and b) the draft?... if the performance changes make for a better sailing performing boat then go ahead... if the draft is but a few inches then go ahead hell it could be increased a foot and compared to other boats of a similar nature it wouldnt be too deep... as long as it retains the trailerability and better sail performance then I say go for broke!!

Of course this will mean I have to toss the keel Ive made up... sigh... BUT on the good side no worries I can use it elsewhere!! and thats a good thing cause so far its simply been driving me knuts a few boxes a nice sideboard its all good

Cheers
and Im glad they got through... so aside from all that above the plans package isnt to bad is it! very thorough in the details and explainations... and the deadeyes make it soooo salty it gives you sunburn

Howdy Dingo,

The changes will increase the draft probably around the 3 to 4" mark - it won't be that much.

But don't scrap the keel - the extra depth can just be laminated on the bottom. and the curve recut.

The bottom panel will need to be rejigged slightly in length to make everything work.

Consider replacing the wheel with a tiller so you can move more forward in the boat when steering - that will also help get the weight out of the back.

We are talking a lot of performance here!

But I need some information about a realistic weight for the beestie - can you plunge into their forum and see what you can find out. I'd rather work with real data rather than something like the following estimate I just worked out.

The ply sheets come to around 300 lbs so I would guestimate
ply - 250
timber, glass, glue - 250
rig - 60 and sails
fittings and bits and pieces - 80
So around 640lbs.

This could EASILY be out by 20% either way.

starts to make the displacement of the original design 404lbs look pretty sick particularly when there is another 350 lbs of crew for an all up sailing weight around 990 lbs.

Does the boat have ballast? That would be extra.

But an actual trailer weight would be great to get.

MIK

Rightio... thought Id already done this last night but seems Im having some issues with the forum software... (server too busy at 2am??? get a grip... but then I seem to be unable to get on at all for at least several periods a day with the data error page coming up)

Anyway...

1) No ballast

2) Weight of boat dry as designed with rig 600 - 700lbs

3) include trailer 1000 - 1500lbs depending on trailer make style and whatever else influenced them at the time

Re Ballast... Some have made a lead shoe to fit over the keel of up to 250lbs or in one blokes case a lead bulb of roughly the same weight... others have made a box and installed it inside the cabin and this gets lead bags of anywhere between 160 - 250lbs some like it that way rather than fixed so that it could be moved with trim but thats more rare from my reading most make it fixed

Tiller... not an issue I have both a wheel and a tiller the wheel can easily be made into a sexywexy clock and the tiller can be reshaped to fit easily.
Cheers

Originally Posted by Wild Dingo

2) Weight of boat dry as designed with rig 600 - 700lbs

I'm not bad am I - estimated 640lbs above!!!

Re Ballast... Some have made a lead shoe to fit over the keel of up to 250lbs or in one blokes case a lead bulb of roughly the same weight... others have made a box and installed it inside the cabin and this gets lead bags of anywhere between 160 - 250lbs some like it that way rather than fixed so that it could be moved with trim but thats more rare from my reading most make it fixed

Do you want to do that? I'd have to add it to the rocker - and it would make the boat more seaworthy.

Tiller... not an issue I have both a wheel and a tiller the wheel can easily be made into a sexywexy clock and the tiller can be reshaped to fit easily.
Cheers

What about making the wheel into a hat?

MIK

ACtually Ding, you blessed canine ...

Have a think about the ballast. It will make the boat a bit more laborious to get up on the trailer etc.

But it would add to the safety factor - do these boats ever go over? How easy are they to get back up again?

MIK

Originally Posted by Boatmik

ACtually Ding, you blessed canine ...

Have a think about the ballast. It will make the boat a bit more laborious to get up on the trailer etc.

But it would add to the safety factor - do these boats ever go over? How easy are they to get back up again?

MIK

aaaahh the question no one wants to admit to!... Given the constant references that they "are tippy" and "would go over" "have no floatation" hat have come up on the forum over there over the years... although Im yet to see someone say directly "my weekender turned into a turtle today" Id say mmmm yeah! Easy to get back up? probably maybe along the lines of a small cat? although probably more difficult due to lack of floatation ergo rather full of the liquid stuff!!

Although Id say people would stand on the keel and with halyard in hand walk up the hull as they pull the masthead up... of course this entails some superhuman effort on said keel/hull walker but doable Id think... more so if theres a couple of bods.

Id rather the lead if you can mate... safety bein me middle name and all and internal flotation if possible would be good Im thinking... now about that transom end of things... now Im the kinda bloke that likes some nice curves on the bum you know? so Im wondering how difficult hard it would be to turn the transom into a nice upsweeping round stern?

Longer doesnt matter a hell of a lot to me as I will be using the underbody from an old 24ft caravan (with a few mods as needed) for the trailer anyways so if it stretched out a tad thats okay but that transom just doesnt look right to my luggerish mind so a nice upsweeping curved bum... now lets look at that stem... mate what about we bring that out and straight up like say... well like a luggar you know?

oooh Im bad man Im bad as!!... can you see what Im doin here huh can yer?? I WILL GET MY MINI LUGGAR!!

A wheel hat?? mmmm Im thinkin here... may have to make some wee modifications I think... but well doable!! Talk about makin that Capn Sparrow (Pirates of the Carribian) look a right woos!!

Looking at the pics of the boat Mik cant see that its 24' long even with the bow sprit.
The rig on that particular boat would not be that expensive with its set up looking at it all galv turnbucles shakles etc poly rope for the stays so it would not be to expensive to use that type of rig
As for dragging its transom in the water looking at the pictures that appears to be the design .By the way our boats transom sits in the water it does not effect its performance [as Daddles has been sailing in her, he can confirm this]as this is its design its sister boat at Port Adelaide Sailing club same design of hull just built in aluminium wins most of the races at that club every year so if its designed correctly wheres the problem. As for no stays etc the rig on Charlie Fisher is unstayed but is an expesive sentup because of the aly masts and type there of.
So what have you got against BERMUDA rigs apart from the cost

Better the a sub just re attach wings and there youy go

Originally Posted by STEPHEN MILLER

Looking at the pics of the boat Mik cant see that its 24' long even with the bow sprit.
The rig on that particular boat would not be that expensive with its set up looking at it all galv turnbucles shakles etc poly rope for the stays so it would not be to expensive to use that type of rig

Compared to what I guess. I've done the maths too many times working out the prices of jibbed boats - there is a lot more hardware required compared to something that doesn't carry a jib.

I agree - choose the level of technology you want and set up the rig that way. If you want to go for polytarp sails any rig - both the freestanding rig and the bermudan sloop will both become cheaper.

As for dragging its transom in the water looking at the pictures that appears to be the design .By the way our boats transom sits in the water it does not effect its performance [as Daddles has been sailing in her, he can confirm this]as this is its design its sister boat at Port Adelaide Sailing club same design of hull just built in aluminium wins most of the races at that club every year so if its designed correctly wheres the problem.

Sorry, you are against the whole knowledgebase of boat design. Go ask any reputable designer on the planet. The lower the power and the lower the speed the bigger the difference.

The the depth and the width of the transom both make a lot of difference - if the transom is immersed the shallower and narrower it is the better. Maybe your boat doesn't have much immersed transom in both width and depth. A hint - in light winds move the crew forward so the transom is out - you will go faster.

If I was drawing up a sailing or rowing boat and had to accept some degree of transom immersion to get the displacement and other numbers that I wanted I would try to keep the difference between the displacement where the transom is just touching the surface and the designed immersion down to 10% of displacement. This is just a number I'm picking out of the air because I know the less there is - the better it is.

The weekender has a difference of 100% between these two figures. Even more if we are talking about ballast.

Immersed transom increases drag at low and medium speeds
Basically if the transom is immersed there will be a swirl or turbulence trailing behind the boat. It takes energy to create that turbulence. The boat can only generate a certain amount of power from engine or sails. Lose some in turbulence and it is energy that could be used to propel the boat.

Look over the back of your boat sometime - get rid of the swirls by getting the transom above the water and there is more energy for driving the boat.

The effect is immediate and obvious with smaller racing dinghies. Drag the transom and you get passed - move a bit further forward and you match speed. Instantaneous and obvious - same displacement - same waterline length - same sail area.

Immersed Transom reduces nett drag at higher speeds
Interestingly it changes a bit when the boat gets up to planing speed. An immersed transom reduces drag by increasing the lifting force on the hull. Theres a lot I could say here about the pressure distribution along the hull and how the breakaway increases the vertical lifting force acting directly on the hull but it would take too long.

Oh - b*****r it! Here goes...

At the front and back of the disturbance around the boat the water has to be at the same level. At the front the water is being pushed down and out. The two sides push out in equal amounts so those effects cancel. The water being pushed down under the boat has an equal and opposite force that pushes the boat upwards reducing its displacement. ie the boat's bottom will experience a positive pressure above the non moving state.

At the back the energy is reversed - the water moves back up to the surface level - to make this happen the forces operate in the opposite sense - so the back end of the boat is pulled down into the water. Negative pressures.

If the transom corner is out of the water these positive and negative forces are, as near as spit, equal. There are some losses but they are minor - I can explain most of them as well if you like.

But if the transom is in the water and the boat is going fast enough for the water to break smoothly away from the transom and continue the hollow the boat is travelling in a few metres behind the boat then a lot of that negative pressure is not pulling the boat down any more. So you end up with more force pushing the boat upwards than pulling it down.

So rowboats and small sailing dinghies have transoms at or slightly above the surface and if they are immersed pains are taken to keep the transom narrow and/or shallowly immersed.

AND planing powerboats - which are godawful to row or sail because of their wide deeply immersed transoms - look at all those swirls at subplaning speeds are very efficient at planing speeds.

These things happen for a reason - so we design boats different ways for different speed regimes.

Summary regarding transom drag
The amount of immersed transom has to be carefully calculated for different speed regimes.

Most "normal" sailboats spend most of their time at speeds less than hullspeed so it is worth their while to have little or no immersed transom or if immersed it is shallow and narrow.

As sailing vessels get faster - cats and 18ft skiffs - the rules are changed. They are optimised toward speeds higher than hullspeed and they get around the excessive transom drag at lower speeds by having both narrow transoms (compare 18s of the early 70s with their big wide sterns to the modern ones and the difference is obvious) and relying on moving the crew a long way forward in the light stuff to sink the bows and get the bum out.

Some higher speed ocean multis and monos can use water ballast to get the weight forward but with the increased sophistication of weather routing extensive computer modelling is done to get the degree of immersion optimum for the mix of speeds expected.

Boats that operate almost purely in planing regimes have well immersed transoms in terms of both depth and width.

Rig costs - apples with apples

As for no stays etc the rig on Charlie Fisher is unstayed but is an expesive sentup because of the aly masts and type there of.
So what have you got against BERMUDA rigs apart from the cost

The Charlie Fisher (norwalk Island sharpie) setup is at the high end of both cost and sophistication for unstayed rigs. You may also note that I don't specify alloy spars on any of my own designs for this reason.

Don't put me in the position of defending a particular boat - I have nothing to do with them these days - the last bit of work was to design the yawl and ketch rigs for the little cat rigged NIS18.

But if you do the quote to compare a sloop rig with a similar level of equipment the unstayed rig is so much cheaper it is a joke.

If both have wooden spars - it is cheaper.
If both have untapered alloy spars - it is cheaper
If both have tapered alloy spars - it is cheaper
If both have carbon spars - it is cheaper.

But only for similar levels of equipment. If you are talking about a sloop with galv turnbuckles, well, I'll downscale the equipment level on the unstayed rig that I'm comparing it with too and still undercut you.

There ARE some disadvantages to unstayed rigs though - the major one is it puts the weight of the mast up in the eyes of the boat where there is a serious increase in pitching moment.

I've done the quotes both at Duck Flat when the NISs were first being developed and for three years in retail chandleries spending a lot of time quoting different rig and sail combinations for a vast range of boat sizes.

And look how the cost of the number of sails and the amount of gear on a sloop multiplies as soon as racing rears its "ugly" head.

A caveat is that I don't know what NIS boats / Spunspar are charging for the mast/tabernacle arrangement these days. But with the tabernacle you are paying for a level of convenience that a sloop just can't match - the NIS's if set up properly can rig in a few minutes - <10?

But like I said - thats the top end if you don't count carbon - compare like quality with like quality and the freestanding rig will almost all the time be cheaper.

MIK

Er, I mean ..
Best wishes
MIK!

And dont you go gettin Mik started!!

ever felt like this?



No seriously... hows things goin? I mean hows yer day? Bloody ducks knutz of a day over here in the West

Ar Mik you are a wealth of info sometimes getting you riled up is a good way to extract it in a direct easy to understand form.
Every little bit helps
I have attached photos of our boat out of the water the water line is about 20mm below the antifoul line and dont mind one of the photos has been coloured in with blue texta didnt it tosee how the hull would look in Royal Blue before we repainted her in that colour.
When we sail we get no swirl as you say out the stern form the transom, flow is straight out , the boat is well balanced moving crew forward makes little difference 200 hundred litres of water up front balance up 10hp of Bukh diesel sail drive up back, 4 people in the cockpit does not lower the transom into the water anymore then one does .
The keel is all steel with 750kgs of lead on the bottom, rudder skeg and rudder are all steel as well, hull is garbon ply Dynel sheeted as is the deck and cabin.

Ar Mik you are a wealth of info sometimes getting you riled up is a good way to extract it in a direct easy to understand form.
Every little bit helps
I have attached photos of our boat out of the water the water line is about 20mm below the antifoul line and dont mind one of the photos has been coloured in with blue texta didnt it tosee how the hull would look in Royal Blue before we repainted her in that colour.
When we sail we get no swirl as you say out the stern form the transom, flow is straight out , the boat is well balanced moving crew forward makes little difference 200 hundred litres of water up front balance up 10hp of Bukh diesel sail drive up back, 4 people in the cockpit does not lower the transom into the water anymore then one does .
The keel is all steel with 750kgs of lead on the bottom, rudder skeg and rudder are all steel as well, hull is garbon ply Dynel sheeted as is the deck and cabin.
Dingo its lovely and wet over here at present and the sun is shining at present

Oh dear Stephen!

20mm below the antifoul line means you've got bugger all transom in the drink. See up there when Mik said apples with apples?

The boats in question have 100 mm or so of their entire flat transom submerged.

Cheers,

P

There is a 100mm of transom in the water that antifoul line is about 150mm above the the lowest point, the transom is at the waterline is around 1200mm, beam is 3metres LWL is 9.5 metres, draft 1.075
and I thought apples were oranges mate, I know its not flat across as in the transom as the Stevenson boat I havent seen the plans so I dont whats below the water.
But Mik can explain the difference and where the hull shape makes a difference in this case