Combine Power output or not?

[Inerested]

Hi Everyone
I am very interested in designng and building of powerboats, but don't have any experience to draw on. I have read a number of books on designing of boats but none of them have answered a question that I have.

Assuming you have a powerboat with a displacement hull, obviously this hull has a requirement for a certain amount of power to acheive a speed of say 8 kts. ( I am ignoring weight etc to make it easier. Although I realise that this affects power requirements) If the power requirements for this hull speed of 8kts was 100Kw ( Sorry I am Australian and am not well versed with Hp) I assume that you could place a single engine in the boat of 100Kw to acheive 8Kts. If you instead decided to use two engines over a single. Could you place two 50Kw engines in and still achieve 8kts. Or would you need to place two 100Kw engines in to acheive 8kts.

I suspect that you would need to use two engines that could provide more than 50Kw, to acheive the 8Kts. However I don't understand what the reasoning is behind it. Can anyone enlighten me??

[mmd]

In general principle, if you double the engines, you would halve the power to maintain similar performance. In reality, though, the niggling details will toss that assumption into the trashcan. Firstly, two smaller motors will probably weigh more than one big one, so you will need more power to overcome the extra weight. Next, you are doubling the number of items hanging off the bottom of the boat, thereby increasing the appendage drag of the hull which will require more power to overcome. Finally, you will have to evaluate the efficiencies of the smaller twin props versus the single larger one to ascertain whether they are capable of transmitting the same amount of thrust per horsepower input as the big single. Given the broad assumptions inherant in the above response, I would expect that a bit more horespower than half of the original single motor would be needed in a dual engine installation to maintain similar performance. It is a more complicated topic than it appears to be at first glance.

[Portager]

I agree with mmd, the weight and drag will tend to increase your power requirements. One benefit of dual props is if the geometry and draft are limiting your prop diameter then two smaller props can be more efficient than one prop a the same diameter.

Another consideration is the level of performance that you want or need under single engine operation. You may need more than half power to overcome currents, wind and wave resistance.

The real question is why do you want twin engines? Cost and maintenance nearly double but reliability improvements are minimal, especially on diesel boats. If you want maneuverability while docking, you may want to consider a bow thruster.

Cheers;
Mike Schooley

[tom28571]

What they said above, but:
Planning engine power requirements for a displacement boat around capability to give 8kts in calm water will leave the boat seriously underpowered. There are many situations that require more power than this like headwind, waves and maneuvering in inlets, etc. Dave Gerr's book, "The Nature of Boats" will give you some good insight in these things.