Inboard prop question.

[gandrfab]

Ya it's a glass boat. I think the members here can help a friend of mine out.

26 Shamrock, 410hp GM motor, straight inboard.
Fixing to order a transmission for my boat.

I currently have a 71C with a 1.52:1 gear. I have a 16 x 18 prop under the boat.

I am going to buy a 72C (rated for the hp I have).

I have enough room to go up to a 17" prop and have a 17 x 18 prop now. I am considering going to a 1.91: 1 gear and going to the larger diameter prop, because everything I read says it will plane at lower speeds and cruise more efficiently.

The easiest thing to do is just go back with the same gear and same prop, but now is the time to change it if I am going to.

Do y'all think I'll actually see enough improvement to make it worthwhile.


I'm wondering if I will notice enough difference for the aggravation of changing. I can't find anything that will tell me how much of a difference it will make.

Will the boat plane .25 mph slower or will it plane 5 mph slower.... Will the economy at cruise improve .05 gallons/hr or 1 gallon per hour....

If the change will be noticeable, I'll change now. I'm just curious if it will be enough to notice.

[gandrfab]

More information


Duckman asks
"How about contacting the transmission MFG.... They may have standard calculation programs using hull weight / HP / RPM etc....

Boats with keels are quite a bit different than outboards and stern drives... more diameter usually helps them get the blades out into undisturbed water."

Boat owner replies
"Been on the phone with a transmission guys several times.

It goes like this.

How much difference do you think it will actually make? Oh, it will make a difference, but I don't know how much.

Do you think it will be enough to justify changing the gear ratio? Well, it might, but it will be difficult to tell for sure.

If it were your boat, would you leave it the same or change it now? I can't really say without knowing what the boat felt like before.

If you had to guess at the improvement in economy at cruise and lowering the planing speed what do you think the change will do? It'll do something, I just don't know how much.


I guess I'm just going to have to guess....."

[kmorin]

g&r, I've paid for membership at boatdiesel.com for years in order to use their online calculators. http://boatdiesel.com/

IF your friend is considering spending money of the class these items costs, this site's cost per year, membership payment required, allows use of the prop calc, the engine power calc and they take into account LOA, Displacement, Hull type (V or semi), Engine Horses, Reduction gears, Prop pitch and diameter..... so he could get some decent theoretical figures instead of "I dont' know" on the phone.

Are any of these calculators exact? NO. But if you entered the two different scenarios for engine, gear and props, you'd get a set or comparative graphs that may lead him to more information than he's got now?

Cheers,
Kevin Morin

[kmorin]

G&R, just about when I hit the "submit" button it occurred to me that Dave Gerr has published The Propeller Handbook http://www.amazon.com/exec/obidos/ASIN/ ... 20-3189700 and that has a series of tables, nomographs (sp?) and other calculations for just the type of questions you're trying to resolve.

D.Gerr's book can be found all over the place but one of the least expensive places I find is alibris.com a used book, online source. I've purchased from them many times buying 20$ books for 1.00$ on more than one occasion, so if the book route is considered ? a potential low cost source is Alibris.

cheers,
Kevin Morin

[gandrfab]

Any thoughts on a 4 or 5 blade ?

[kmorin]

Gee-n-Arr,

http://www.stingrayboats.com/products/r ... 03_06.html some tables listing a well known plastic boat test series?

Cheers,
Kevin Morin
Kenai, AK