tumblehome

[dreamer]

I've been trying my hand at drawing a powerboat. The goal is not so much to have a super speedy hull. The main goal is to make it comfortable in a seaway and as efficient as possible. I suppose 'semi-displacement' is where this will fall.

What effect would tumblehome have on the performance of a powerboat? My first impression is that it will have very little impact except if the boat were rolling. In that scenario, tumblehome should increase stability as the boat rolls (think canoe) though perhaps it will be minimal.

ALso note the chines. I showed these to another designer and he mentioned that my chines made the bow look a bit 'weird'. Apart from aesthetics, would there be anything inherently bad about thes chines as I've drawn them?

[Joe Petrich]

Hi Rick,

Could you post a set of lines? That would make it easier to see what you have.

There's nothing wrong with a chine on a semi displacement hull form. In fact you want one forward as a spray knocker.

It looks a lot like a planing hull as it is now. You may want to make the hull a little deeper with a steeper run and more v in the sections. Sections below the chine are generally more rounded in a semi displacement boat as well. See the attached image.

Best regards,
Joe

[dreamer]

Thanks for the reply Joe. 'Spray Knocker' is what I was thinking with the chine.

Attached is the lines I put together.

Are you saying I need more deadrise for semi displacement?

[Ad Hoc]

Dreamer

If the boat is fully planing, then once the water has been separated from the hull via the chine, what is above has no influence, in terms of speed. (Unless your ultra high-speed and looking at air resistance).
The tumblehome will affect rolling if you think the boat will roll lots or wish to use tumblehome to reduce rolling. But the effect will be minimal if the tumblehome you're look at, for styling, is around 5~10degrees.

Look at some of the old classic Riva's...

[FAST FRED]

Only usual knock on tumble home is it reduces the deck area aft a bit.

Not a problem on a sweet looking boat.

FF

No, "displacement" is where this will fall. Go away from semi for what you want. Nice lookin', tho, IMO.

[Joe Petrich]

Rick,

You could add a bit more rocker to the hull while keeping the tarnsom/hull intersection in the same spot. You may want to make the sections more convex than concave. What's the half angle of entry? It looks OK.

With the chines sucked so far inboard the waterplane will be comparitively small meaning the boat may be sensitive to weight and loading. Also the roll will be a bit softer at rest and at slow speeds compared to a similar size boat with the chines further outboard. That could a good thing but the boat may also feel a bit tender.

I think mark755 is right if you are going for a really efficient boat. Personally I like long narrow boats with efficient propulsion packages. One of my favorites is Dashew's "Wind Horse", though I might go for a single screw version with a get home drive.

[dreamer]

Thanks Again Joe.

I was wondering about rocker too I notice the Grand Banks models have quite a bit.

Curious re: narrow boats. Yes, I know what you mean for sailboat hulls, but don't we want high initial stability with powerboats? That isn't typically attributed to narrow boats. As they don't have the deep draft that a Dashew boat does (will need to read more about Windhorse).

Except, I guess, when they're moving (gotta stop thinking 6kts is fast )


Steve, I was initially shooting for displacement. After looking at more hulls and your and Joe's comments, I'll make some modifications for this to be more displacement-y.

[Joe Petrich]

Yes the older GBs are oriented to the displacement end of semi displacement and you will see considerable rocker in them. I have always liked the 32 for its efficient 1 GPH operation.

I think most powerboats these days have high initial stability because they have wide beams for more room and the horsepower to overcome the associated drag. But that's not efficient. A narrow boat can overcome it's tenderness with fin stabilizers and/or rolling chocks. It is interesting that "Wind Horse" has fin stabilizers and a truncated fin keel. I am not sure if the fin is there for ballast or to provide protection for the stabilizer fins in case of grounding. Probably both. Of course experience has proven that you rarely hit the rock dead on.

[PAR]

Tumblehome was incorporated to provide foot space while working the aft decks (dragging gear over the side). It's too high up the stern sections to have any bearing on stability, within reasonable powerboat angles of heel.

Your lines have a few things I wouldn't incorporate into a semi displacement form, while others I wouldn't use on a full plane form.

Currently she looks to be a warped bottom form with a fairly fine entry and unfortunately hollow WL's forward. I can't tell what the true path of the chine is, but it appears it may be tucked in aft a little too much.

At this point there's not enough information provided about the hull form. Length, target speed, displacement, power available, a conventional lines arrangement, etc. would be helpful.

If you're looking for displacement mode only, this hull is all wrong shape wise. As far as semi displacement you could make adjustments to this hull and as a full plane again some adjustments, though she's more full plane than anything currently.

[dreamer]

Thanks tons for the feedback. This is all very useful. Stay tuned for more...

[Tad]

Volume distribution in any hull....sail or power....is dependent on the design speed. Nothing realistic can be said about your form without a length, target speed, and LCB location.

Ideal LCB for full displacement speeds (S/L 1.4) is approximately station 5.3-5.4, ideal for semi displacement speeds (S/L 2.2) is approximately 5.6-5.7, with a speed length ratio of 3 or above, LCB at station 6 or over is best. For more on this see http://www.tadroberts.ca/about/pdf/p...d-function.pdf

Looking at this form I would say she'll be a real "snow plow".....too blunt forward and fine aft for anything above full-displacement speed.

Once you get the LCB in the right place for the speed, you have to get the weights to work as well. Then give her enough power to move at design speed....it's simple!

[dreamer]

Originally Posted by Tad

....it's simple!

most encouraging qoute yet.

[dreamer]

Joe,

I've spent some time now looking over Dashew's design. I like it. 'Unsailboat' pretty much fits the description. The shape makes sense for a full-displacement vessel.

[Joe Petrich]

I guess I skirted around the numbers issue. Ishouldn't have. Tad's article describes the inter-relationship of the various numbers better than I ever could.