Hull strenght, how much more for 40 mph, 50 mph, 60 mph, 70 mph?

[seabuddy]

Is there any info about how much stronger one has to design a fiberglass 21'-26' bow rider boat to give good service life as the top speed increases?

If you are re-powering such that the top speed will go from 48 mph (250 hp I/O) achieve roughly a 67-71 mph speed by using a 525 hp I/O how do you design for the increased stresses that the hull will encounter at the higher speed?

Bayliner adds another layer of cloth and reinforces the hull/deck joint for about the last 6' of the sides and across the transom for their model that goes from the mid-40s to the same model, but optioned with enough power such that the boat now has a top speed of 58-61 mph.

Do you know how others can come with the proper lay-up for other cases/situations?

[Eric Sponberg]

Hull strength is dependent on speed, and there is no easy remedy like add another layer here and there. The pressures on the hull go up with the square of the speed, and the entire structure--hull skin, bulkheads, frames, stringers, etc. all work together. You really have to re-engineer everything in order to build in the strenth for higher speeds. This is a complicated process of calculation for which you really nead a survey of the original structure, and some engineering time from a reputable engineer to determine where more structure is required.

Eric

[seabuddy]

Thanks for your reply. I understand that in broad terms, a hull strenght increase in the order of a ball park of 400% would be appropriate for a hull going from 45 mph top speed to 65 mph top speed.

I understand that its a more tech answer than this before you actually build the boat.

[Eric Sponberg]

Loads increase with the square of the speed. So from 45 mph to 65 mph would give an increase of 209%. This does not mean that you need 209% more material, rather you have to design for the loads, and there are ways of increasing strength and stiffness without necessarily increasing the thickness of the hull laminate.

One way to increase strength and stiffness is adding more frames. Strength and stiffness are inversely proportional by the square and cube respectively as the length of a hull panel or frame decreases. That is to say, if you have a frame that is 2' long, and you brace it with a bigger frame crossing it such that it cuts the frame in half to 1' long, the frame will be twice and strong and three times as stiff. So when engineering the boat, you really should look at the whole package of structure to see what should be beefed up.

Eric

[seabuddy]

What a fact filled answer!

I thank you very much. It was nice that you took the time to do some cal. and come up with the 209% answer.

Thanks again.

[sorenfdk]

At these speeds, you should use longitudinal stiffening.