250HP outboard Speed prediction for heavy boat

[bydgroup]

Hi Nacaman!

We have currently in the design board a new motor yacht of 51 ft and a displacement around 12.5 tonnes and are thinking on using 2x300 Hp outboards, to achive a speed of 20-22 kts. All numbers seem to be right and pwer prediction as well, but never had the experience with a heavy boat with outboards.

As I've seen that you had the same question I would like to ask you a few things..

Did you had the chance to test your boat? Do you have some conclusions on the use of outboards for heavy boats? Did you found the appropiate propeller.

The engine manufacturers say that is not an usual thing so they dont want to confirm or deny the performance, and also I'm concerned on the durability/relayabililty of the engines!!

Thanks!

[nacaman]

Hi Bydgroup,
The boat is under production but not tested. This whole area of the outboards for heavy displacement boats is a minefield due to the lack of technical data on outboard prop performance. There is a far bit of uncharted water here. From a technical standpoint an inboard with prop installation would be a safer and more robust long lasting installation, albeit with higher initial costs. If the operating hours were higher, the diesel would in the longer term reduce this cost difference. In our case hours were lower and client did not want to pay the extra initial cost.
For the vessel we were originally intending to use 2x250 hp engines. After discussing this with outboard manufactures, we got various opinions and responses.
The complications we came across were

1) The thrust loading on the props is high and much higher than would acceptable for a commercial prop. Some cavitation is likely, which may affect prop life. I have read this should not cause thrust breakdown, but it is uncertain how much effect this will have on the thrust generated. The most suitable outboard appears to be the higher reduction ratios with the biggest props, however none we found were fully satisfactory for blade loading. Despite this the warm fuzzy feedback we received from outboard prop suppliers was that the speed was possible to achieve with these outboards
2) It appear the speed prediction for a heavy vessel based using outboards is best determined by assembling lots of old salts at the pub, plying them with drinks, and comparing against performance of other unrelated speed boats and averaging the answer.
3) We were unable to obtains data on the propeller efficiency for outboard at our proposed trial condition. Other guys I talked to were using ballpark QPC's of 0.4-0.5 for props. In reality this will vary with different boats and speed. Further the outboard manufactures we talked to, could not match a vessel resistance curve against the performance of their outboard to predict a vessel speed. Again comparison against there trials data was done, the data we saw did not include heavier planing boats.
The prop size we looked at marginally above Gerrs suggested limits for minimum prop diameter for maneuvering

Operationally
We have ended up increasing the power to 2 x 300 HP outboards. This was based on feedback from the outboard manufacturer that outboard g/box on the 250 HP was not robust enough to do the job and would prematurely fail. We also find out that as outboards are generally intended for pleasure boats that some manufacturers assumed only around 120 hours per year of operation. This may be fine for a pleasure boat, but could be a concern for more commercial operation.
The path is a bit risky and if you do go down this path, there are uncertainties regarding the final vessel performance. If you can get the outboard manufacture to agree to change the outboard to a larger outboard if it fails to perform, it would be a worthwhile assurance to have.

[bydgroup]

Thanks Nacaman for your comments!!!

I would love to hear your impressions once you test the boat with the outboards.

On the meantime we have not received from any manufacturer any thrust data, the only ones that answered us with some helpful comments is Mercury, that they believe that their mercury verado 300 will do the job

Those are some of their comments:

On a normal V-bottom (this boat appears to have a flatter bottom) planing hull at 13.5 tons with 600hp total inboard power, the speed calculates at 30 MPH. With Zeus/600 HP, the speed would be approx. 36-37 MPH.
With twin 300 Verados, the predicted speed would be about 27 MPH; however, it's important to note that you will need 12" pitch propellers. These props are only available in right-hand rotation, thus you would need both engines to be right-hand rotation.

The 300 Verados consume 28-30 Gallons/per hour each at WOT (wide open throttle). At cruise,(3000-3500 RPM) the consumption is 10-11 GPH.
The Verados will work well at 9 and 20 knots.


Regards

[powerabout]

Asking an outboard to do the job of an inboard diesel is asking for trouble.
Just look at the gear ratio and the prop sizes and you will learn that big outboards are not for pushing houseboats.
Your lefthand gearbox will not like all that load even if you can get a prop?
Prop slip will be huge.