power needs

[BTScow]

Can some one point me in the direction of calculating the power needs for my 18' boat I am designing. My plans (subject to change) are to power my craft with an 16 hp inboard diesel with a 2:1 gear reduction. I would like to estimate my cruising speed. I have not determined what size prop I will be turning yet, so I am open to suggestion.

Many thanks in advance.

BTScow

[robherc]

Hmmm...I guess I'd start with putting the hull into freeship, use the drag calculations from there to see how much power is needed for a certain speed, or how much speed is possible with a certain power (and, best of all, freeship is free)

[Willallison]

Well, to start with, you'd need to know the boats' cruising displacement - ie its total weight with fuel crew and stores / gear on board.
If it's a displacement boat (and with only 16hp it most likely will be unless its very, very light)
Then, I'd recommend you get hoold of Dave Gerr's Propellor Handbook. All the info you need regarding estimating speed / power and propellor selection is in there.

[Guest625101138]

Originally Posted by BTScow

Can some one point me in the direction of calculating the power needs for my 18' boat I am designing. My plans (subject to change) are to power my craft with an 16 hp inboard diesel with a 2:1 gear reduction. I would like to estimate my cruising speed. I have not determined what size prop I will be turning yet, so I am open to suggestion.

Many thanks in advance.

BTScow

A 16HP diesel in an 18ft boat sounds like it will be heavy displacement. Guesstimate of weight might be 1.2t. Doubt that you will need more than 10HP to get hull speed of 5.6kts. Anything more will be wasted in making bigger waves for very little speed increase.

If you provide more idea of the intended hull shape then it is possible to accurately determine the speed for a given power and the most suitable propeller.

Rick W

[PortTacker]

I think your best cruising speed is gonna be just back of hull speed, so find the rpm that engine is most economical at, and prop accordingly?

[tom28571]

Most here seem to think you are limited to something like "hull speed" with a 16hp diesel and a 18' boat. It ain't necessarily so. So much depends on the boat and how it is set up.

For instance, Robb White claimed 20 knots with his 20' Rescue Minor with a 20hp 3 cylinder diesel. You won't get that, but you don't have to settle for displacement speed unless you want to. The power is plenty and the main issues are weight and hull design.

[Riverrat1969]

Don't let the nay sayers get you down. I have a 24 foot flat bottomed scow (Carolina Skiff) powered with a 9.9 HP four stroke Evinrude (Suzuki in disguise) outboard.

This boat is set up as a "swamp boat".........extreme skinny water, full of snags, so no need for speed. With eight adults, two large dogs, and lot's of gear, we cruise at fast idle speed at 3 statute miles per hour (GPS) and top speed of 8 MPH. The last trip out, I took note of fuel useage...........16 MPG, mileage by GPS.

You have to get the correct propeller, for good performance. With the stock propeller, reverse was almost nonexistent. With a low pitch, wide blade prop, reverse will stop forward motion with a burp of the throttle.

Jim

[gonzo]

You are pointing at the most important aspect. He has to show what kind of hull and what it weighs to get a reasonable answer.