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Old 24 January 2008, 08:17   #11
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Does this not make something like the Yam FT9.9 high thrust ideal for the job. Only 10 HP but you will be pretty much guaranteed to get the whole 10 HP.

Bedajim - what weight is your 15HP suzi?

Richard
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15 2 st Merc = about 36 kg + 25 L fuel in the back of the boat
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Old 24 January 2008, 09:14   #12
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Running some figures through a prop calc [
.....but does it take into account the massive windage of a RIB? Ever tried paddling one off a shore against a moderate wind? Into a decent headwind the power to drag the hull through the water will be swamped by that. You can guarantee the main won't break down on a windless day...

Doesn't someone do stoopidly low pitch props for canal barge outboards & the like?
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Old 24 January 2008, 10:38   #13
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Originally Posted by 9D280 View Post
.....but does it take into account the massive windage of a RIB? Ever tried paddling one off a shore against a moderate wind? Into a decent headwind the power to drag the hull through the water will be swamped by that. You can guarantee the main won't break down on a windless day...

Doesn't someone do stoopidly low pitch props for canal barge outboards & the like?

Canal barges don't tend to use outboards from what I've seen-they have stupidly low pitch props on big lazy diesels. Effectively what the riverboats using outboards have is a sail drive prop.

Look at it this way. An engine has to rev to the recommended WOT range to produce the stated HP at the prop. Increasing the stated HP at the prop won't make any odds unless the engine can rev high enough to get to the stated HP-or has enough HP that far down the rev range to make the boat plane.Until then it's only going to either (depending on throttle setting)
a) create a lot of prop slip
b) run at the HP needed to get the boat to the hull speed
c)Cavitate.
d)Labour badly.
If it's possible to get a stupidly low pitch prop like that then yes, it's feasable to run a 25hp-but people are far too hung up on HP. What you need to move at displacement speed is a higher torque figure at the prop-and you can get that with a motor that produces significantly less HP but is propped (geared) correctly.
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Old 25 January 2008, 05:54   #14
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Thanks for all your replies and its comfirmed my gut feeling about a 25hp beeing thirsty and not geared correct against a smaller lighter geared correct engine which also will give more econimy because am am thinking of the worst case having to go 10-20 miles so to carry two petrol cans for those rare times.So i shall sell the 25hp and look out for a 15hp or simular thats built or capable of high thrust. Feel free to carry on if anyone can suggest anything suitable they have or can be re propped to make more suitable
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Old 25 January 2008, 06:13   #15
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Peter, I have just dug out a 1995 Mariner brochure so here are some details of 2 strokes for you to ponder over, don't think the 2 strokes have changed much over the years. Weights are for the standard length, so couple of extra kgs for a long shaft. I have a Mercury Seapro 10 hp which is just about liftable if you were bobbing about at sea, although I would want a rope around it just in case.

6 - 9.9 hp, 210cc, 2.00:1 gearbox, prop pitch range 5.5" to 9" and 31 kgs.
(Standard prop is 9"x9")

10 - 15 hp, 262cc, 2.00:1 gearbox, prop pitch range 6.5" to 10.5" and 32 kgs.
(Standard prop is 9"x9")

20 - 25 hp, 400cc, 2.25:1 gearbox, prop pitch range 9.5" to 18" and 51 kgs.
(Standard prop is 10.4"x13")

Pete
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Old 25 January 2008, 08:18   #16
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Nos, I was being a little figuritive with the use of "canal barge" as a generic heavy displacement craft, but my theory still stands. Any engine will move anything evventually in ideal conditions - I towed 2 8 metre yachts through a flat calm with my Suz 25 last summer - took forever to get them going and for the first few mins probably at about 90% slip, but they moved.

I ran a pitch calculator on the Pirhana website for a displacement transom hull, and they reckoned 5-6" for that application and a 25Hp (I assumed the rib weighed 2 tons, being a Pac).

The problem here I think is that no 25 has a prop pitched that low.

Daft question - why not just hang it on the transom & see what happens?
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Old 25 January 2008, 09:07   #17
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Nos, I was being a little figuritive with the use of "canal barge" as a generic heavy displacement craft, but my theory still stands. Any engine will move anything evventually in ideal conditions - I towed 2 8 metre yachts through a flat calm with my Suz 25 last summer - took forever to get them going and for the first few mins probably at about 90% slip, but they moved.

I ran a pitch calculator on the Pirhana website for a displacement transom hull, and they reckoned 5-6" for that application and a 25Hp (I assumed the rib weighed 2 tons, being a Pac).

The problem here I think is that no 25 has a prop pitched that low.

Daft question - why not just hang it on the transom & see what happens?

Fair comment. I couldn't really see another large volume application though. If a prop with a low enough pitch was available then maybe,but when there's 15hp sail drive/bigfoot motors available with low pitch props...
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Old 25 January 2008, 09:43   #18
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Yeah, maybe I should have looked at the "clear english campaign" website before I posted that one!

Agreed there's lots out there probably more appropriate, but if you got a 25 lying in your garage, I'd be tempted to give it a try!

I guess someone could make a special prop, but it will probaly be cheaper to P/X the 25 for a saildrive!
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Old 27 January 2008, 13:45   #19
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Thats the FT9.9 on order for an aux. Will report back on how well it performs.

Richard
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