optimal relative transom width for low power planing hull skiff?

[BE_]

BE, none of us intend to be picking on you. That said, it appears that you are more interested in what the boat will look like than your interest in best and most proven lines for a boat of that type. That mindset is a familiar one here on the forum. You seem determined to have the boat to look like what you want it to look like. I say go for it then.

Please do not take offense for what is written above. I repeat, we are not here to give anyone a hard time.

a flat bottom plywood skiff is a flat bottom skiff, a v bottom aluminum boat is a v bottom aluminum boat.

i just want advice on how to design a flat bottom plywood skiff, not what v bottom aluminum boat i should be buying instead

 

[Barry]

BE, none of us intend to be picking on you. That said, it appears that you are more interested in what the boat will look like than your interest in best and most proven lines for a boat of that type. That mindset is a familiar one here on the forum. You seem determined to have the boat to look like what you want it to look like. I say go for it then.
Please do not take offense for what is written above. I repeat, we are not here to give anyone a hard time.

I am trying to design a small (12'x4') ply-on-frame flat bottom skiff that can plane on somewhere around 6-8 horsepower and go about 10mph. Would it be better to have the transom be nearly as wide as the widest part of the hull, or significantly narrower to achieve this? I keep seeing people say both.

a flat bottom plywood skiff is a flat bottom skiff, a v bottom aluminum boat is a v bottom aluminum boat.
i just want advice on how to design a flat bottom plywood skiff, not what v bottom aluminum boat i should be buying instead

The original posters query was to find out if a wide or narrow "transom width" or chine width would be better to enable planing at low horsepower. It was later that the OP said that the purpose was to build a boat of a particular shape and
never again mentioned the parameters that he introduced in his initial post.

I introduced the picture of the Lund as it was the target length of his sketch but exhibited those design elements that have been proven for 3/4 of a century, whether is is built of aluminum or wood or fiberglass. No one is suggesting
which aluminum boat to buy.

Lund boats have an almost zero deadrise in the wetted surface area when planing. So it is not a V hull.

So, going back to the first post, if you are trying to attain a reasonable speed with an 8 - 10 horsepower flat bottom hull, a wider chine width with parallel chines in the aft 2/3's will be able to provide the best speed or load carrying capacity
than a narrow chined boat.

With the change of goal to build a boat with your sketch with the very narrow planing area, performance MAY be compromised/unobtainable.

With the original parameters AND including the new desirable shape, and accepting a compromise on performance, then build your "shape" but use a wider chine width.

As Messabout said, no one is trying to give you a hard time or suggesting which boat to buy, but rather some basic design parameters to help your build

 

[gonzo]

The drawing shows no rocker at all. That is likely to make the boat broach when running in even small waves.

 

[BE_]

The drawing shows no rocker at all. That is likely to make the boat broach when running in even small waves.

i was wandering about that. i was copying jeff spira's carolina dories (Spira International Inc - Pescadero Carolina Dory https://spirainternational.com/hp_pesc.php), which have no rocker, or even an upturned bow. i like this because it makes it easy to build, but would raising the bow 4 or so inches make a big difference in performance?

 

[BE_]

upturned the bow, increased flair in the bow, and widened the chines

 

[BE_]

The original posters query was to find out if a wide or narrow "transom width" or chine width would be better to enable planing at low horsepower. It was later that the OP said that the purpose was to build a boat of a particular shape and
never again mentioned the parameters that he introduced in his initial post.

I introduced the picture of the Lund as it was the target length of his sketch but exhibited those design elements that have been proven for 3/4 of a century, whether is is built of aluminum or wood or fiberglass. No one is suggesting
which aluminum boat to buy.

Lund boats have an almost zero deadrise in the wetted surface area when planing. So it is not a V hull.

So, going back to the first post, if you are trying to attain a reasonable speed with an 8 - 10 horsepower flat bottom hull, a wider chine width with parallel chines in the aft 2/3's will be able to provide the best speed or load carrying capacity
than a narrow chined boat.

With the change of goal to build a boat with your sketch with the very narrow planing area, performance MAY be compromised/unobtainable.

With the original parameters AND including the new desirable shape, and accepting a compromise on performance, then build your "shape" but use a wider chine width.

As Messabout said, no one is trying to give you a hard time or suggesting which boat to buy, but rather some basic design parameters to help your build

is this a better shape?

 

[Barry]

is this a better shape?
View attachment 177592

My opinion is yes, I think that the rear upper beam width dimension ( at the transom) would be more than 44 inches if the mid chine width is also 44 inches. So increase this to scale keeping the 44 mid chine width 44

 

[BE_]

My opinion is yes, I think that the rear upper beam width dimension ( at the transom) would be more than 44 inches if the mid chine width is also 44 inches. So increase this to scale keeping the 44 mid chine width 44

like this?

 

[Barry]

like this?
View attachment 177594

Looks pretty good, but it might be worth accommodating a bit of an upturned rocker, only at the front to somewhat mitigate Gonzo's concern. If you are in light wave conditions, it should not be a problem but if you are caught out in
stronger conditions and nowhere to hide, a bit of front rocker may help.

 

[BE_]

Looks pretty good, but it might be worth accommodating a bit of an upturned rocker, only at the front to somewhat mitigate Gonzo's concern. If you are in light wave conditions, it should not be a problem but if you are caught out in
stronger conditions and nowhere to hide, a bit of front rocker may help.

this is probably it for revisions, i'll start making the model soon

 

[messabout]

When running at less than planing speed some front rocker will make steering more certain. If the forefoot is allowed to be too deep, the boat can be twitchy in even a small chop.

 

[Dolfiman]

Again about curvature : if you plan a ply-on-frames construction, you should pay attention to the curvature variation, the less variation, the easier the building. To have in mind the flexure of the ply panels. For example, the Lund 12 showed above would be uneasy to do due in ply due to the peak of curvature of the sheer line. From a given drawing, you can always try to minimize the curvature variation without too much geometry change.

 

[messabout]

When you build the model, you might have some excess twist in the side near the forefoot. If you have the stem raked at the most convenient angle the side panel will lay nicely without twist. Eyeballing the drawing it seems a bit too much rake. In any case, you can build to your last drawing with confidence that the boat will work as hoped. The 8 HP engine will drive it fast enough to be plenty of fun if lightly loaded.