Whats the largest auxilary i can consider

[PeterR]

I am fitting out my Pacific and fully intend to do many long cruises and have an auxilary of a reasonable /large size 2 stroke even as big as 25hp but am unsure in reality if this would work out well.Any comments wellcome

[Pete7]

Pete, I think the trips Chris Kaye made in the early 90s would probably serve as a good example. His rib Sabredrive was wide beamed Delta 6.5m rib fitted with a mid engine Perkins 160 hp and an experimental surface drive, so a similar sized rib to a Pacific 22. On one of his attempts to reach Iceland he broke down. They made it safely back involving a return trip of 90 miles (or possibly 90 hours memory fading) on a Yanmar Diesel outboard at displacement speeds, so its certainly possible. He was successful on his third attempt btw.

I think your going to need a long shaft to get the prop down deep otherwise it will break the surface as waves roll uner the hull.

Pete

[Poly]

Originally Posted by PeterR

Any comments wellcome

good cause I ain't no expert!

I think the challenge with too big an outboard is it still won't get you on the plane (unless you go silly!) but the prop/gearbox on most 25 HP engines will have been designed for most small lightweight boats at planing speeds.

If you are still only managing 7 or 8 knots at full throttle then the prop must be slipping like mad (or the engine won't have enough torque to get up to full speed). Not sure if you can get the gearbox and prop combo you need to get full power delivered effectively.

The diesel outboard pete refers to presumably has gearbox and or prop which enables most of the power to be delivered, and higher torque?

What extra would you gain with a 25 HP engine over a 9.9/15/20 in this application?

[Nos4r2]

Personally I'd get the biggest sail drive outboard I could. Does anyone know if the Yamaha 9.9 that was easily converted to 15hp was available with a saildrive prop?

[9D280]

For the 25s Suzuki go down to 9" pitch and Tohatsu can sell you an 8" for their 25s.

That's just the info I have to hand. Maybe Merc / Yam go lower?

[Nos4r2]

Originally Posted by 9D280

For the 25s Suzuki go down to 9" pitch and Tohatsu can sell you an 8" for their 25s.

That's just the info I have to hand. Maybe Merc / Yam go lower?

I think this is one that might need some trials to find out-but a 25hp isn't going to get a pac22 planing so you're looking at displacement speeds.

I reckon a 15hp with a sail drive prop would do the job. It'll still do the same speed but it'll use less fuel.

[9D280]

True, but realistically how many Aux's will get their boats planing? At a rough guess - none! The only way that's going to happen is a pair of big low pitch prop'ed O/bs (on separate tanks filled from different pumps etc etc). If youpre doing that, the low pitch you;d need to plane on one will eat fuel in "normal" cruise, and will of course be slower, not what you want for the rest of the time.

If you're running a 25 over a 9.9 you will have a bit more grunt to get you through a big headwind / waves. You don't need to run it flat out all the time!

The Aux is there to get you out of trouble.

[Nos4r2]

Originally Posted by 9D280

True, but realistically how many Aux's will get their boats planing? At a rough guess - none! The only way that's going to happen is a pair of big low pitch prop'ed O/bs (on separate tanks filled from different pumps etc etc). If youpre doing that, the low pitch you;d need to plane on one will eat fuel in "normal" cruise, and will of course be slower, not what you want for the rest of the time.

Agreed!

Originally Posted by 9D280

If you're running a 25 over a 9.9 you will have a bit more grunt to get you through a big headwind / waves. You don't need to run it flat out all the time!

In theory you're right, but in practice it won't work like that.
Running some figures through a prop calc (link):-

With a 9" pitch prop
1.85 or 2:1 gearbox ratio
5000 rpm (which is low)

theoretical top speed from a 25hp engine of 21 or 23 mph.

Dial in the hull speed (or a realistic speed you could push to past the hull speed)-I dialled in 8mph- you're going to get 62% slip. Your 25hp is never actually going to get to the right number of revs to develop 25hp unless it's cavitating-and it'll still use more fuel than a 15hp.


For example, to make a genuine 8mph at 20% slip using 2:1 gearbox ratio and a 9"pitch prop your engine is going to be doing 2347rpm. At that speed it's unlikely to be developing 15hp.

Originally Posted by 9D280

The Aux is there to get you out of trouble.

Yep, agreed. It doesn't make sense to put something on there that's heavier,juicier and no faster though.

[bedajim]

Originally Posted by Nos4r2

Agreed!
In theory you're right, but in practice it won't work like that.
Running some figures through a prop calc (link):-

With a 9" pitch prop
1.85 or 2:1 gearbox ratio
5000 rpm (which is low)

theoretical top speed from a 25hp engine of 21 or 23 mph.

Dial in the hull speed (or a realistic speed you could push to past the hull speed)-I dialled in 8mph- you're going to get 62% slip. Your 25hp is never actually going to get to the right number of revs to develop 25hp unless it's cavitating-and it'll still use more fuel than a 15hp.


For example, to make a genuine 8mph at 20% slip using 2:1 gearbox ratio and a 9"pitch prop your engine is going to be doing 2347rpm. At that speed it's unlikely to be developing 15hp.


Yep, agreed. It doesn't make sense to put something on there that's heavier,juicier and no faster though.

I get 8 knts from my 15 2st(prop as fitted new), bit of air in a big swell but I may make a new bracket over the winter

The 8 knts happens from about 3/4 revs +, with the main down as a rudder but it would get you home

[Richard Selman]

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

[bedajim]

Originally Posted by Richard Selman

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

Hi
15 2 st Merc = about 36 kg + 25 L fuel in the back of the boat

[9D280]

Originally Posted by Nos4r2

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?

[Nos4r2]

Originally Posted by 9D280

.....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.

[PeterR]

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

[Pete7]

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

[9D280]

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?

[Nos4r2]

Originally Posted by 9D280

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...

[9D280]

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!

[Richard Selman]

Thats the FT9.9 on order for an aux. Will report back on how well it performs.

Richard