Modify Sailboat to Semi planing

Have you seen my Tunnel cat I have just built.Its in catamarans and multihull gallery. Warped V low speed planing.It might not be wow for most people but I being an amateur had to build it 3 times to get it right and I am stoked with the outcome.It obviously could be tweaked further but she slides through the water at idle and starts planing at low speeds.Built the lines by feel of what i thought the hull would do no calculations just looked at a lot of hull shapes.Sharp V's forward to cut the chop and bigger planing warped V aft to shallow dead rise at transom to compensate for the zero lift area in the tunnel.I see many boats out there (New designs) relying more on power for lift than hull shape. That's why I am Mad about Boats and designs, I believe low power planing is key to future power boating and this is where I want to learn more even though they were designed years ago time and speed has become more important (small powerboats pocket cruisers 18ft) than low power semi planing (displacement or whatever the term is for it)

 

Not again !!

Now that's a really good way of putting it !! and when they do get the bare facts shown to them without frills they usually get up set ,pop there thumb in the mouth and head for a dark corner wishing mummy was there , "he's been nasty to me !! "

 

Doug, by coincidence, I happened to click on "Newest Gallery Images" under "Most Recent" at the top of the forum page and I saw your tunnel cat but didn't notice that it was yours. It's cool. Congratulations.

Since you already have talent and ability, you will only get better with increased knowledge.
I look forward to your future contributions.

 

I like Doug's attitude a lot. He is undoubtedly a man that applies himself, with perserverance, to a task. But I would say this, even if he straightened out and filled in everything from midships back, which is seemingly what he intends to do, he would not have a "semi-planing" hull, but a planing hull, and it would probably need 40 hp at least to plane comfortably or economically. 20 or 25 hp would be running at max output, and bad fuel figures. But I'd say it will slap pretty good on the flattish bottom amidships. Then there is a question of what will happen when he turns to run with the waves, with all that new-found bouyancy aft and no keel to keep it on track. It will admittedly be a two-trick pony rather than just a 5-6 knot plodder, but the low-speed performance won't be near as good as before surgery. It will not perform at all well in the transition speed range of 6-10 knots, but up and running at 12-15 knots should be OK. But 20 or 25 hp won't suffice imo. Being relatively light works in your favour, but the additions to the hull have to be factored in, and is it built to withstand slamming loads on the underbelly that will be greater than at sedate speeds ? Interestingly, the picture Doug posted of the modified hull with a box keel has a V4 OMC engine of at least 85hp fitted. Don't know what they were thinking was the advantage in such an experiment.

 

You are into tunnels ??
wow !!
that's where my head is at 25 hrs a day 8 days a week .
Its your design ??
Long time back I worked with and for Scott Robson !!
Race boats ,off shore and F1&3 tunnels were what we were into all day long plus a range of family boats as well all tunnels , I learned so much while I worked there it was incredible .
its the most amazing part of boating very few people really ever come close to getting it any where near right , I also had a lot to do with a guy that was a off shore tunnel boat driver and he taught me heaps as well about how to set up motors and how best to time and handle tunnels . I will never forget those days!!, its stuck with me for the last 20 odd years .

 

I think he could have a semi displacement hull, IF (big if) the aft sections where shaped properly. He could keep his 20 HP and get moderate speeds. Instead of just dragging the lines aft and squaring off the stern sections, say about 2 - 3 degrees of up sweep, drawn out as sweet as practical, would put the immersed portion of the new displacement fairly close the LWL at the transom. This is a long way around the bend for an S/L of 2.5, but a fair sight better than 1.5. Basically, this means after all this work is done, you've got a boat that has moderate low speed manners and the ability to move at 11 - 12 MPH, which is much better than 6 MPH. High speed maneuverability can be handled with a skeg and the substantial rudder that will be left over from the displacement speed requirements.

The after sections shaped like this, also means much less volume is gained with the modification. A weight study to account for the new hull sections, the engine and related elements (tanks, controls, etc.) should be done, so you can get a good grip on how much volume you actually need. With this, you can shape these sections with a Cp more in keeping with the speeds she'll move at too.

 

Hi all,After reviewing all said and checking the lines of Ninigret, Lb22 and Tophat I have resigned myself to the fact that it is not going to work and must be satisfied with 6 knots and the possibility of trimng the nose down only with ballast and a wedge at the back.

So I am now considering that only, I know that I wasn't happy at those speeds on my previous 2 so I am back to square 1,I might have to just build from scratch.

The Tunnel cat is a great stable fishing boat but unprotected so my wife is not happy she wants a cabin boat.

Yes Tunnels, I designed and built from scratch couldn't find the shape I was looking for so crashed on built one from Styrofoam with 4mm ply skin to see if it would work It was Ok but the tunnel was a bit small not enough to swallow a small chop so re-drew the lines got the frames cut on the CNC at work for accuracy and built the second one upside down on frames the conventional way.I had the original deck running over the tunnel but didn't like the feeling of sitting so high.So cut out and re-shaped the internal frames so that you straddle the tunnel.Much more comfortable with this setup and fishing is great being able to move up and down the sides of the boat. Watch this space I will put some pic's up soon of the layout when it stops raining.

 

I just deleted someones private message by mistake please re-send.

 

Did you find a lines drawing of the boat Doug ?

 

Another reason for being cautious about "butchering" a Hartley TS16 is the cult following the vessel has acquired, any deviation from the othodoxy could be seen by the more rabid afficiandos as grounds for justifiable homicide.

 

That has worried me to.I am re-thinking and looking at building from scratch.So looking for plans for a 18ft boat that runs with a 25hp. Ninigret is a bit long for trailering and launching by myself.Probably good to start a new thread for my quest.I like the little Ryds 485 runs great with a 15hp so a version slightly bigger will do the trick.

 

Why not just suffer the indignity of slower speeds ? The family will find it a more agreeable motion anyhow, than a planing boat. And do a lot of trolling, that way you can have an interesting side attraction while going out and back.

 

Yes that id an option I am looking at.The internal berths on the boat are not good due to leaking windows so need to re-build the internals anyway. It is a good candidate or the modification as the trailer is well built in good nick and can easily extend the draw bar if the length increases to 18ft. Still mulling over all options haven't found the lines to build a model yet.

 

The plans are readily available, including used ones occasionally. Give them a call and see if they'll sell model making plans or just the lines.

 

Hi All just got back from work yes this is my other problem Time. This Boat building is only done after hours so have to be clever.

As I see it these are my options.
1. Modify the hull to a planing hull and up the motor to a 40hp if necessary.
2. Use the boat as is with the 25hp and live with 6knots.
3. Look for a longer donor boat.
4.Find plans for a low speed planing boat happy with a 25hp 2 stroke at 10 knots cruising or 12knots topend or better about 18ft trailerable.I like the Ninigret but 22ft on a trailer launch myself not sure (Any sugestions appreciated)Maybe start a new thread for this quest.