250HP outboard Speed prediction for heavy boat

I have been looking at a proposed project for a 14m x 4m landing craft. Although inboards would have advantages, client wants outboards. Loaded boat is around 14 tonnes displacement and they want to go 20 knots. One unit proposed produces 2 x 250 hp outboards at 5500 rpm thru a 2:1 g/box ratio (ie 2750 rpm at the propeller). A 15" diametre propeller has been proposed with anywhere from 11" to 16" pitch (depending on who is asked)

One area I have struggled with is trying to find actual design data from outboard suppliers on what thrust the outboard prop actually produces at different speed or what the prop efficiency might be. It appears the solution proposed is to turn up with a bag of propellers when the boat is in the water and try to get the rpm right, so that the maximum outboard power can be achieved (this might be OK at the other end of the build, but doesnt help much for current issue )

Other than checking there is enough power, my concern is the propeller may be too small for the job and excessively cavitate. However Im not sure where I might look to find actual thrust or other prop design data on the outboards or the outboard props.
Anyone got any suggestions or suggest where to look ?

 

Outboards have better power to weight ratio's than inboards.

What does your landing craft looks like ? 14 tonnes is a lot of weight for 20knots.

 

You seriously need a NA

it just aint gonna happen

somebody is smoking thier socks

 

landing craft aand outboards

Look over Aluminum Boat Plans by Specmar, Inc. they show lots of landing craft of all sizes and weights--all out of aluminum. They have some unique craft. Just some info for you. Stan P.S>. Yamaha offers HP high power high
torque models--their 10 HP will outpull a 20-25 HP model of other manufacturers.

 

PropCalc (google Castle Marine) tells 16 1/2" x 11 1/4".

The boat is just in the limit of planing, Froude number 1.0. Use deadrise under 15 degrees and straight lines. Could be possible.

 

Women weigh themselves without their clothes on so they weigh less (true) . Is the landing craft painted

 

To do a more accurate design you need to provide some detail on the hull shape. As a first approximation the weight to drag on a low speed planning hull is around 8. So at 14t the drag will be a bit under 2t. Allowing for some fouling and protrusions lets say 1t (10kN) per engine. A flat bottom will plane more readily for example.

The attached shows what you can expect from a 15" prop. It will need to be pitched 15 to 16". I would go 15" in the first instance. The efficiency will be 68% and you will require 150kW or 200HP. So the 250s are OK for power.

The concern I would have for this application is the thrust capacity of the outboard leg. Outboard motors of this size are not intended to go this slow near full power so the thrust loading is likely to be well above design. Think about that tiny prop being constantly loaded at 1t. I have heard of heavy duty commercial legs in the past - maybe you need to investigate this.

Rick W

 

I should add that going up in diameter improves efficiency. A 16" prop gets close to 70% and pitch drops to 14".

If you could swing 18" then efficiency 71% and pitch just over 12".

Ultimately it gets down to trial and error to get the best prop for the application because there are many factors involved. As an example you may not be able to trim the boat to design for the given load conditions. Another is that the surface finish may not be fair or is not always clean.

What I have provided should be a good starting point but you really do need to talk to the outboard makers regarding the application. It is certainly heavy duty. Each motor with a 15X15 prop will be capable of producing a peak thrust of 20kN at 13kts providing the leg bearings can transmit the force.

Rick W

 

Here is the Savitsky prediction for the boat in flat bottom version. It is very sensitive to weight distribution according to Savitsky. If the weight is forward it will tend to dig a big trench and will take huge power to get over the hump.

The numbers for the conditions shown are close to my first approximation for drag being weight/8.

Rick W

 

Thanks for all the feedback. The hull is intended to have a shallow vee around 7 degrees, with a small flat chine outboard. The Vee will remain fairly shallow up to near the bow, so that the ramp can be fitted.

 

Savitsky indicates a slight 'V' reduces drag a little. Surprised me actually.

Rick W

 

Flatter displacement.

If you want the boat to be able to do 20kn then in my opinion it must be able to do so reliably. If you have to do it downwind all the time you may not do it too often.

Also, if you have the motor work flat out all the time it is going to affect the life of the motor for sure. If you overpower a bit and you can do the same on 70% power you will save on fuel too and the motor should last twice as long.

 

http://www.billmunsonboats.com/

 

Using that Savitsky calculator needs some caution. It seems to calculate Lk and D clearly wrong. Lk is important, since this vessel might easily have insufficient LWL for keeping the bow section out of water. Savitsky method is not valid then and the calculator gives no warning.

However, the thrust seems to be OK and it can be as low as 16 kN with 7 degree V at 20 kn. That means only 160 kW of propulsion power. With propeller efficiency of 70% 2*160 hp should be enough to maintain that speed.

Joakim

 

Thanks, I can´t find the savitsky calculator to play with....