relative power from 2 versus 1 outboard engine

Discussion in 'Outboards' started by jedkins, Aug 17, 2007.

1. Joined: Jul 2006
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 9
Location: phuket

jedkinsJunior Member

I have a 20 ' sailing trimaran with 1 , 3 hp outboard mounted near the stern on the starboard side. It powers the boat along at about 5-6 knots. For cruising/ no wind situations, I am considering mounting a 2nd , 3 hp outboard on the stern port side. Does anyone know what sort of speeds I might achieve with both engines running.?

2. Guest625101138Previous Member

The hull speed for a 20ft boat is 6kts. This is not quite a brick wall on a narrow hull in the same way it applies to a wider hull but it becomes a steep climb. I could guess and say 6HP will give 7.5kts.

If you give me the waterline beam of the main hull and total boat weight I can give a more informed estimate based on calculation. If the outer hulls are heavily loaded then these would also come into play but neglect them at this stage.

Rick W.

3. Joined: Jul 2006
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 9
Location: phuket

jedkinsJunior Member

Thanks for input, The DWL beam of main is about 2' and the 2 outboards would be about 3.5 feet apart.

4. Guest625101138Previous Member

Any estimate of the total weight??

Rick W.

5. Guest625101138Previous Member

I think my first guess was way out. I have played around with a hull that I think would be good for a tri. Something like a trailer tri in this article:
http://www.f-boat.com/pdf/News54.pdf

I tested a model using Michlet and it gives the result shown in the chart attached. It shows that 3HP will give 7kts. I have made allowance for losses but the hull might be better than yours. Maybe the outrigger drag is significant.

The point is if you go to 6HP you should expect about 9.5kts (Maybe 9kts if you hull is different to the model). Sounds too good to be true but if you read through the trailer tri article you see that it does 13kts with 9.9HP so Michlet looks close even though it is getting on the plane. I have used a flat aft section so there will not be much sinkage.

You can see the wave drag kicking in at 5.5kts but the broad stern and narrow hull allows it to get up and plane reasonably easily.

Rick W.

Attached Files:

• tri.pdf
File size:
24.2 KB
Views:
286
6. Joined: Jul 2006
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 9
Location: phuket

jedkinsJunior Member

Total weight of boat including engine and gear is approx 400 kg.....

7. Guest625101138Previous Member

I was working on more weight than this for the previous numbers.

You might find that the prop you have is not well suited to the boat. One problem with just adding another motor is that the prop pitch may be limiting the speed so adding another motor just means the motors work less.

Do you have an pictures of the boat that you can post here?

If you can get hold of a larger motor for testing then this could be a better option because it will likely have a bigger prop.

Rick W.

8. Joined: Jul 2006
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 9
Location: phuket

jedkinsJunior Member

here is a picture...

File size:
106.4 KB
Views:
676
9. Guest625101138Previous Member

That looks like a slippery craft. Should go well in a good breeze.

The wave drag should not be very significant with such narrow hulls. The outboard hulls look to have a reasonable load so this adds a lot more wetted surface for the displacement that increases viscous drag.

You could probably do a simple test to check this out by loading one outboard hull enough so the other lifts clear of the water. Compare speeds at full throttle between the case where there is no roll and the situation where you have one hull clear. Or have you already done this?

If viscous drag is dominant then doubling the power will increase the speed by 40%. So if it is 6kts now then you get about 8.5 kts with 6HP.

You may be able to improve light air performance by raising the outboard hulls if both are carrying load. Aim to just balance on the main hull.

Rick W.

10. Joined: Jul 2006
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 9
Location: phuket

jedkinsJunior Member

Thanks for your advice. I guess I will give it a try as might prove worthwhile when I need to do long motoring stretches. I already have a spare 3 hp so just need to make another bracket for it.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.