correct prop size

[bsmit24]

If you are starting from scratch, how do you determine the ideal prop size? I mean either designing a hull or a new boat with no engine or other restrictions. I have seen several articles on this subject but they all dealt with displacement hulls and I was interested in planning hulls. What all do you need to know or assume to get started?

Brian

[Eric Sponberg]

Brian,

First off, you need to know the length of the boat, its projected displacement, and its intended speed. From that you can determine the amount of horsepower you need to go that speed (Crouch's formula). From that you can select which engine or engines you want (engine catalogs). Presumably, you are considering inboard power because outboards come with propellers on them.

At some point, you have to conceive of the general appearance and shape of the boat (your drawings). From that you can get an idea of the hull shape near the propellers and begin planning for the appropriate propeller diameter and pitch (see Dave Gerr's Propeller Handbook which has various methods to determine propeller diameter. I prefer the Bp-delta method, it is very accurate). That gets you to square one with the first determination of propeller size.

In my work, I run a very versatile program called NavCad, which is the best speed/power program available for a PC computer. I use it to give a final determination to the propeller design, diameter, pitch, number of blades, amount of cup, and can even match to various propeller manufacturers brands.

I highly recommend Dave Gerr's book, it gets you really close to what you need.

Eric

[bsmit24]

I am considering an inboard. I have always had outboards but I am looking for something different and have been trying to educate myself on inboard design and setup. I say that I like to fish but sometimes I really think that I enjoy working on my boat as much as I use it.

[PowerTech]

"I really think that I enjoy working on my boat as much as I use it." If that is the case I would recomend to you a wooden boat With A TIB detroit diesel or a KAMD volvo. :D

[crash]

hi guys.... i have two boats hulls, 1 is a 16` waterski boat, and the other is .... well to be honest, i dont know... the previouse owner said it was a powerboat? dunno... anyway, i was wondering if you could help... i need to get an outboard for the waterski boat. she`s a john broom 16`, the height of the top of the transome from the floor is 20". do i need a shortshaft or something.... i dont know much about boats, etc, so any help graetly appreciated. the boat doesnt weigh much... 3 of us picked it up and put it on a trailer.
http://img.photobucket.com/albums/v380/kev/boathull.jpg

[crash]

this is the other hull, any help with i.d. would b great! thanx, kev.
http://img.photobucket.com/albums/v380/kev/bigboat1.jpg

[cyclops]

The other posting confirms it is a freshwater offshore powerboat. Keep land in site at all times and you should have no problems. Enjoy riding it.

[crash]

thanx mate!