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I have 2 Honda 20 hp outboards on my Pahi 42, but can only get 4.5 at 3000rpm, my preferred motor sailing revs.  I have tried high thrust props to get more speed, but only get 4kts, I assume because of the reduced pitch (the lager diameter obviously does not offset this).

I am now considering Yamaha 9.9 high thrust motors with the much larger prop and higher engine gearing (2.9 to 1).

my cruising weight is 6500 kgs.

Does anyone on the forum have any experience in this area of engine performance and any advice to offer.  I have wasted many $ on the so called high thrust props.  To my logic, reduced pitch means reduced thrust, despite the diameter and blade area increase.

Help please!!!!!!

John Wilkie

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Hi John,

I have a Pahi 42 that had 2 Honda 15 HP with high thrust props. With clean hulls, calm seas and lightly loaded I could get between 6-7 Knots out of her. Just last month I upgraded the motors with 2 Mercury 30 HP four strokes and now I can reach about 10-11 Knots with the same conditions. My motors are mounted mid ship on both sides of the helm and are lowered up and down when needed. I hope this helps...Thanks, Craig

With two 9.8 tohatsu on my tiki 38 i get 8 knots full throttle with two engines, 6.5 with one. By calm sea of course.



Thanks for that Craig, It's good to get some real life experience rather than thoughts & ideas.  I think I could probably get 7 kts at full throttle out of my Honda 20's, but I never use full throttle, much to noisy and fuel thirsty.  I am thinking of getting the Yammy 9.9 high thrust props (11 3/4 " diam x 9 4/3" pitch), cutting off the cavitation plate and fitting them to my 20's.  It's all about getting the best speed at 2500 - 3000 rpm so we can talk and drink red wine in relative peace!

Again many thanks for your advice,

Maybe we could swop pics of our Pahis.  My email is svkatfish@gmail.com

Cheers,

John



Hi Jacques,

What speed do you get at say 2800 rpm on one engine, which is what I motor sail at, so I can still talk without shouting and drink red wine in peace!  And what weight is your Tiki?

Many thanks,

John

I'll chip in my Two pence worth on how i understand High Thrust props and motors..

The "normal" thoughts for outboard manufacturers is that their engines go fast through the water with a light ski boat style vessel. If you Load up that light ski boat with 6-7 tons of stuff then you wouldn't be surprised if it doesn't go as fast. The prop can't get the boat unto speed therefore the prop spins in the water, like wheelspinning a car.

High thrust engines and props are more designed to push heavy loads, like a barge, slowly. Therefore they have a different gearbox and propeller pitch that should be geared more towards vessel speed and revs, think torque. No doubt there is complex maths behind all of this which is way over my pay grade.

On my Tiki 46 i have 2 x 25 hp high thrust yamaha 4 strokes. in flat water i get about 5 knots with one engine and around 7-8 with both at around 3700-4000 rpm. If you put my engines next to a yamaha 20hp 4 stoke they are massive compared with the 20hp. The gearbox is probably twice as big and so is the prop. If i increase revs to 5500 i go very slightly faster but burn twice as much fuel. I can't get the engines to rev to 6000 rpm as i believe the props are too course. Its like trying to take off in 5th gear.

I would suggest getting yourself down to a good yamaha dealer and asking to explain what is the difference between high thrust and normal thrust. I don't think its as easy as just changing the prop as the engines and gearboxes are designed to do different things... Think light weight racer motorbike or a torquey ( is that even a word?) tractor. both could have the same engine but massively different gearboxes ands wheels will achieve a different end result.

Cheers

Marty

 

I dont know how many rpm when full throttle (5000?). Let say at half throttle on one engine i get 5, with two i get 6. Boat is 3.5 tons.

John Wilkie said:



Hi Jacques,

What speed do you get at say 2800 rpm on one engine, which is what I motor sail at, so I can still talk without shouting and drink red wine in peace!  And what weight is your Tiki?

Many thanks,

John

What Marty said ^^.

I agree with Marty, on his prop analogy.

My old Honda 15 HP's were very load at full throttle, while the Mercury 30 HP's are very quiet. At 3,000 rpm(6,000 rpm full throttle) you can hardly hear them. I use my Pahi 42

 for charter down here in Mexico. Sometimes towards the end of a day sail we will loose our wind and will need to stay at full throttle for an hour or two to make port before dark. That's the local Harbor Masters rule on charter boats, back by dusk. :-(

While motor sailing, I can't here them. I can't believe the difference compared to the Hondas.

I would also add that if you are using a two stroke then most of the power is only generated at very high revs. If you are only opening the throttle 50% then you aren't using 50% of the power, its probably more like 30%. This also applies to 4-strokes but to a lesser degree. If your prop is too course, i.e. designed for a speed boat to go fast, then it will stop the engine revving to its full potential and therefore the engine won't develop the power. If you want a good example look at two stroke dirt bikes, they are driven at very high revs all the time to access the power band. if you try to drive them with low revs they just don't have the oompf. I think you could also use the analogy of a car trying to pull a very heavy trailer which is stuck in the mud and trying to start in 4th gear, its not going to work and neither is it going to work if you put it in first gear and stomp on the gas as the wheels will just spin and won't gain traction.

Do this test, take your boat out and steam at full throttle and listen to the engine noise, then take it out of gear and rev it with full throttle. Can you hear the difference? Now i'm not suggesting that its a good idea to run your engine at absolute max revs all the time but when you put a load on the engine there has to be a drop in revs as the engine is working hard and if its working too hard then it isn't being efficient and is wasting fuel and is incurring excessive wear on the parts, which brings me back to the car analogy, you have to have the right gear for the job you want to perform. If you tried driving all the time around town in too high a gear it would soon be burning too much oil, using too much fuel and would be "kaput" as the germans say, in no time at all.

Just like life, its all about balance...

My own opinion is to go talk to the specialist outboard engine guys and get something that will do the job that you need. Wharram recommends twin 9.9 high thrust 4-strokes for a tiki46 and i went with twin 25hp high thrusts and i'd even consider going bigger. "Its better to have it and not need it, than to need it and not have it" as the saying goes.

Cheers hope this helps.

Marty

 

And... think of huge tankers, they have massive course pitched propellers that spin relatively slowly and they fly around the ocean at 18-25 knots... The secret? MASSIVE amounts of torque... I worked on a large survey ship last year and the propeller had a constant speed even when the vessel was stopped. The speed was controlled only by varying the pitch of the blades... again the motors, which were electric, had huge reserves of torque... 

Cheers

Marty

Thanks for that Marty, if only electric motors were further developed.  I am realising that to get more speed at the same revs (3000), I need a prop with more pitch, given that pitch is the distance in inches that the prop travels forward in 1 revolution.  No wonder the "high thrust prop of 7" pitch was worse than my standard 10 pitch prop. It sounds like a marketing strategy to sell more props.

If I can find an 11: pitch prop I'll let you know how it goes.

Cheers,

John

Hi John

I run 2 Yamaha long shaft high thrust 25Hp on a Pahi 42 with modified mini keels that would add to drag. Check out my pics. I get 5.5 kt at economical revs in flat seas with no headwind, speed drops quickly as conditions deteriorate. My engines are 12 years old. I am sick of dealing with clogged carbies and want to go electronically fuel injected, the smallest Yamaha makes is 50hp in high thrust. I am considering them despite the increased weigh.

You must go high thrust gear ratio with big prop as most outboards are geared for planning hulls. Without this you just burn fuel and make noise as you push against hull speed in a displacement hull. Unfortunately choice in high thrust outboards with the correct gear ratio is very limited.

Bill

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