planing strakes questions.......

[alibi]

Hi

Im building a 20ft v drive ski boat, I have put one planing strake either side of the centreline one third of the way up the hull running from the bow 2/3 of the way down the hull,

will it ride too bow high? because the strakes dont run the length of the hull to the transom, or should i put a smaller set down aft and half way out from the centreline to the transom to give the stern some lift too?

The deadrise is variable starting at 12 degrees at the transom
Cheers

[Eric Sponberg]

One really can't say what the boat is going to do with any sort of lifting strakes on the bottom because too many other factors are in play, namely the overall beam of the boat, its weight and the location of the center of gravity, and the amount of fore-aft curvature in the hull, particularly up forward. All of these things have tremendous influence, and you can't single out lifting strakes, there or not there, as a predominant factor. That is, what may work for lifting strakes on one boat geometry will not necessarily work or have the same influence on another boat geometry.

Basically, you have to do some trial and error testing and see what happens. Personally, in my small powerboat designs, which are in production, I stay away from intermediate lifting strakes between the keel and the chine because they are actually hard to build well in a production boat--have to fill them really well during production, and when they chip and/or break, they become a maintenance/warranty problem. So to stay away from them is because of simplicity, quality of construction, and easier maintenance. And in those designs I have plenty of speed for the horsepower installed (I am not trying to break any records) and I like keeping the hull easily maneuverable and safe at speed in turns. I don't need the lifting strakes, and it keeps production simple.

That said, if you want them, particularly on a custom boat, they work fine if configured correctly, and if the boat is made of wood-epoxy, they are easy to add secondarily. So your question comes down to configuration--what's going to work? Try what you have and see--that's about the only way to do it without going into a towing tank with a model. You can mimic other boats--look at those that are closest to your design (similar length-to-beam ratio, deadrise angle, and weight)--someone there has already done the trial and error before you from which you may benefit. If they have more lifting strakes, you can put on more.

You should also have a pronounced chine/lifting strake where the hull topsides meet the bottom? Do you have that? If not, you should as that will have more influence on the boat getting up on plane in a stable regime than intermediate lifting strakes. You need the chines first, then you can concern yourself more with intermediates. The reason for putting intermediate lifting strakes on is to shorten the time to get on plane and to increase top end speed. So you can start with what you have, see how the boat goes, and then keep adding strakes for improved performance until you don't get any more improvement (time to plane and top end speed). That will be your answer.

I hope that helps.

Eric

[TollyWally]

It certainly gave me a bit more insight into hull dynamics. That is what I really enjoy about this forum, the little bits of information that come your way. Thanks for the explanation.