10 HP Four-Stroke Fuel Consumption

Doug,

I am not sure what type of speed you are looking for, but I think you may be speccing out way more hp than you need. My Olson 30 at 3600lbs has a 2.5hp that at 1/4 throttle does 6.5kn. If you are just looking for low speed work I would think you could reasonably go to a 1hp or even a trolling motor easily (the trolling motor pushed the Olson at 3kn at Wot).

 

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Stumble, thanks. This is extremely preliminary but I'm thinking cat(smaller) or tri(larger) designed for extreme efficiency. Ideally, 15 knot cruise, acceptable 9-10 knots.

PS-Tom-I'm thinking one engine for a tri maybe two if a cat.....+ a get home rig that could also boost efficiency at certain times.

 

at those kind of HP I think you guys should take a closer look at how wind and wave conditions effect your energy needs

just a thought
B

 

As an agent for Tohatsu outboards I can help you with the consumption of these engines as claimed by manufacturer, but all makes are well within about 10% of each other.
Shown in red is the equivalent consumption for two stroke models.

9.8hp 4/S @ WOT = 3.8 liter (1.0 USGal) per hour 2/S = 5.1 liter (1.35 USGal)
15hp 4/S @ WOT = 5.0 liter (1.32 USGal) per hour 2/S = 7.3 liter (1.93 USGal)

Tohatsu does not have a 10hp in two or four stroke but does have a 9.9hp in two stroke that is for all intended purposes the same engine as the 15hp. It runs slightly lower rpm for max hp than the 15 and some carburetor jet changes, thats it.

9.9hp 2/S @ WOT = 5.5 liter (1.45 USGal) per hour

 

This is actuLly a good point. There are some real regulatory differences between a boat with a 9.9 and a 10hp in the US. If you can get around it going with the smaller engine can save a lot in required equipment, registration, and insurance.

 

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Thanks, Wynand-I appreciate the info.

 

I have owned five boats with 9.9 high thrust Yamaha outboards during the last 25 years and can confirm that at cruising throttle you get nearly two hours per gallon

You should do about 10 knots with a 9.9hp on a 20ft powercat. My 20ft Skoota powercat uses a 25hp outboard and does 15 knots. See

www.sailingcatamarans.com/skootas.htm

A question for engine experts is, if motoring a Skoota at 10 knots is it better to use a 9.9 or a 25hp?? I assume the 25 will last longer as it is less loaded, but which has the better fuel consumption??

I have a 2.5hp on my Strike 18 trimaran. Like Stumble I can motor at 5 knots in a flat calm. But as soon as there is any wind/waves the boat basically stops. That's not a problem on a sailing boat as you have sails. But a powerboat must be able to make headway in, say, 25 knots wind and 4ft seas. So don't underpower a powerboat. And remember that at full speed you'll always have have a 15 knot headwind

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

"Ideally, 15 knot cruise, acceptable 9-10 knots."

This will take POWER! not a 9.9 but maybe a 90!

Most of this discussion has concerned displacement speeds , and fuel flows.

15K may require 15 GPH , depending on the boat.

I would advise running a test , have a buddy with a big diesel sport fish , and a large scale tow you.

Measure the force at as many GPS speeds he will tolerate from 2K on up to however fast you can get to.

A hp of engine (gas or diesel) can make about 20 lbs of push.
Yes theoretically the number could be higher , but this is real world , a non perfect match of engine prop and boat speed.

If you want to run fast in big wind and seas , the power requirement can easily be 200% to 300% higher.

This much power is hugely expensive to purchase and operate when underloaded, about 99% of the time.

FF

 

Where did you get that relationship from? What speed range would you claim it holds?

Do you disagree with this fundamental relationship:

Power = Force X Speed

 

I will add to the chorus here and tell I remember my 36 MacGregor with twin 9.9 yamaha 4 cycle hi thrust motors burning about .5 gal hr each at cruise.

The fact that Richard has owned 5 boats powered by these motors confirms my feelings that they are outstanding outboard motors if fitted to the right boat. Twins on the MacGregor were perfect.

Another thought is that I don't believe the long shaft option is that readily available in most manufacturers lines above the 9.9's. This might not be an issue but keeping that powerhead 5" higher from the water on a small boat is a very good thing for several reasons.

Happy to see how much you like my home State. Two coast's each with it's own barrier islands, a shortcut between them through a vary large lake, its own archipelago, coral reefs, two time zones, many outstanding beaches, another Country made up entirely of islands just 50 miles away....

What's not to like?

Steve

 

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Good comments ,Steve with one exception: people who aren't from Fladah may read them!
Luckily for me my parents traveled all over when I was very young and then planted me on our boat in Pensacola, then Pensacola Beach, then Ft. Walton, Mary Esther in the early days. Even lived in Jacksonville ON THE LAND for a while before then and then Miami ,Dinner Key, and Key Biscayne and now Cocoa Beach with a view of sunsets on the Banana River. I've been very lucky to have found paradise. I want to see and photograph every inch of it away from most of the touristas.

 

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That's encouraging, Richard- thanks! I have a question: have you done any experimenting with hydrofoils for "foil assist" on small power cats? Do you think there are some gains in speed/efficiency possible-in other words do you think it would be worth experimenting with?

 

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Thanks ,Fred. When I get to the stage of designing this thing I will have some serious help. I'm not averse to doing all kinds of experiments to get it right during testing but that's a long way away right now.

 

Have a look at my Skoota here

www.sailingcatamarans.com/skootas.htm and the videos

You'll see it doing 15 knots with a 25hp.

It is the sort of boat Doug is looking for, rather than a heavy displacement liveaboard cruising monohull which is probably what you are thinking of

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Yamaha 9.9hp hi thrust? I think those were the babies I wanted (and were specified) for my boat, but are out of production? (unavailable in this region?) IN THE ORIGINAL LOW GEARED form with a big slow revving propeller for displacement type boats...