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Old 25-07-2016, 01:19   #1
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More Prop Diameter or more HP?

I need help making a tradeoff between larger diameter w/ less torque (12” prop on Yamaha T9.9), higher rev, higher torque, (10” diameter on a Suzuki DF20a), or the monster Yamaha T25, 12” prop, lots more torque, medium revs, a lot more weight.

Boat, Maine Cat 38, sailing cat, lightship 8200lbs, design water line 12,400lbs. Twin outboards just inside the hulls in bridgedeck nacelle. water line 36.5’, length to hull beam ~11.4

In a fantasy world I'd like 7knots at 75% throttle on one engine, 10-11knots both engines WOT. I'd like to go with the Suzuki as it's EFI and a number of friends just love the EFI and the efficiency the computer dials in for long range. The local dealer who handles Honda, Suzuki, and Yamaha said the Suzuki EFI has been the most reliable engine in this range by far.

Yamaha T9.9
  • RPM 5500
  • 2.92 gear
  • 114 lb weight
  • Prop RPM WOT 1884
  • Torque @ max RPM 27.6 lb ft
  • 12" max prop diameter
Suzuki DF20a (EFI)
  • RPM 5800
  • 2.08 gear
  • 108 lb weight
  • Prop RPM WOT 2788
  • Torque @ max RPM 37.8 lb ft
  • 10" max prop diameter
Yamaha T25
  • RPM 5500
  • 2.42 gear
  • 201 lb weight
  • Prop RPM WOT 2273
  • Torque @ max RPM 57.8 lb ft
  • 12" max prop diameter
Not sure how to compensate for slender hull. If I put one hull into Vicprop - Propeller Calculator half the weight, one engine, it seems to say that the suzuki should work, it's recommending 9-10" diameter x 6-7" pitch.

A number of articles talk about slip in the 50% range being normal for displacement hulls, so don't think I'll create a milkshake. The big question is how much pitch can you dial in before it just looses grip and becomes useless?

On the other hand, some calcs seem to show that thrust is a function of diameter^3, so every extra inch of diameter should offer huge benefits. I wonder if the T9.9 would get close to the 20hp in real world due to the low speed big prop. Don't really want to weigh the thing down with the T25 unless the T9.9 is just too slow and the DF20a is too high rpm and won't grip.

Lots out there that talks about adjusting pitch given a fixed diameter prop. But haven't been able to find the math that helps be compare these scenarios.

Any guidance on the analysis much appreciated.

Thanks

Mark
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Old 25-07-2016, 09:34   #2
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Re: More Prop Diameter or more HP?

I know that too large a prop can overload your motor. But I'm certainly not an expert in any form or fashion and it would take more data than you or I have to make that analysis. I would expect that the manufacturers would have put many expert hours in to choosing the right prop for their motors. But perhaps not for your specific application.

I have heard good things about this book: https://www.amazon.com/Propeller-Han.../dp/0071381767

Don't know if it would have the answers you seek. I think it would take some serious calculations. Good luck.
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Old 25-07-2016, 09:52   #3
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Re: More Prop Diameter or more HP?

I have the Zuk on a tender and love it, but I do not think it an appropriate motor for a high thrust application.
I've seen Merc 15's high thrust with large diameter four blade props that seemed to work well
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Old 25-07-2016, 10:16   #4
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Re: More Prop Diameter or more HP?

First you need a high thrust version of the outboard. It isn't just the prop size, it is also the gear ratio that changes. Then I would probably start with the largest prop the drive leg can handle. Outboards, even high thrust outboards, are so targeted at small fast boats you almost can put a large enough prop on them for a displacement hull.
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Old 25-07-2016, 10:48   #5
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Re: More Prop Diameter or more HP?

Not on the right kind of boat, but I believe this motor will give you the HP and thrust, without as much weight, what I see I don't know is correct, but it looks like a high thrust 15 is 115 lbs?
https://www.mercurymarine.com/en/us/...on+Redirection

On edit, the pro kicker is 135 lbs, still not 200 though.
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Old 25-07-2016, 10:56   #6
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Re: More Prop Diameter or more HP?

Quote:
Originally Posted by Mark424 View Post
-----
Boat, Maine Cat 38, sailing cat, lightship 8200lbs, design water line 12,400lbs. Twin outboards just inside the hulls in bridgedeck nacelle. water line 36.5’, length to hull beam ~11.4
-----
Mark-
I thought the Maine Cat 38 was being offered with one outboard and a bow thruster. Did this change, or does the buyer have the option?
Thanks
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Old 25-07-2016, 13:26   #7
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Re: More Prop Diameter or more HP?

Quote:
Originally Posted by Stumble View Post
First you need a high thrust version of the outboard. It isn't just the prop size, it is also the gear ratio that changes.
Quote:
Originally Posted by a64pilot View Post
Not on the right kind of boat, but I believe this motor will give you the HP and thrust, without as much weight, what I see I don't know is correct, but it looks like a high thrust 15 is 115 lbs? https://www.mercurymarine.com/en/us/...on+Redirection.
Thanks,

Most of the use of "high thrust" is just marketing (other than the yamaha T9.9). e.g. most standard outboards in the 10-20hp range have a 2.08 gear ratio, the mercury "high thrust" is 2.15, 3.3% difference. Similarly the Honda "Power Thrust" is the same gear, but swaps the 9.25"x 10" prop for a 10"x 7 5/8" prop. Suzuki high thrust 9.9 is the same, they swap in a 10" diameter prop.

Another way to look at it, the lower gear ratio gives you higher torque and lower RPM. But if you have a higher HP motor, you are already getting the higher torque. e.g.

Yamaha T9.9 at WOT. 1884 RPM, 27.6 ft-lbs torque.
Suzuki 20hp at 70%. 1952 RPM, ~ 37 ft-lbs torque.

So the real question comes down to if adding more blades or more pitch to the prop can use the extra torque at that rpm or if the torque is wasted. Secondly, once you bring on the extra RPM, will the power go into the water or will the prop "break free" and just spin.
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Old 25-07-2016, 13:33   #8
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Re: More Prop Diameter or more HP?

It's more of a lower pitched, larger diameter prop thing than it is gear ratio.
My Zuk DF20A will just cavitate if I push it very hard tied to a heavy object, I know as I pulled a couple of people off a grounding in small sailboats when I was down in the Keys, it will push my 3.10 meter dinghy at stupid velocities, but that 10" diameter prop will just slip when asked to pull hard at low speed, its like having a 400 HP car with thin tires, car will go stupid fast, but won't accelerate in a drag race, it will just spin its tires.

Now if you could put a 12" four blade low pitch prop on that Zuk, I bet it would really pull then, but wouldn't push the dinghy at 30 kts either
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Old 25-07-2016, 13:44   #9
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Re: More Prop Diameter or more HP?

Quote:
Originally Posted by FSMike View Post
Mark-
I thought the Maine Cat 38 was being offered with one outboard and a bow thruster. Did this change, or does the buyer have the option?
Thanks
Hi Mike, that was how hull #1 was built, but hull #2 forward have changed to twins like the MC30. I think in certain sea states, there was bow wave interaction and more ventilation than they liked or they would have had to lower the nacelle lower than they wanted.
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Old 25-07-2016, 13:55   #10
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Re: More Prop Diameter or more HP?

I would second the comment for high thrust version motors. They are pretty amazing. two 10HP would move your boat well I would think, but if you may need more HP at limited times is the problem. Turning thru the wind etc in a close anchorage etc. With gusty winds in a tight anchorage with a Lagoon 42 and two Yanmar 3GM30 diesels There were a couple times where I lost maneuverability and almost collided.
For practicality the Suzuki 20's might be a good compromise for the huge weight savings...
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Old 25-07-2016, 14:28   #11
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Re: More Prop Diameter or more HP?

You almost have me curious enough to take a load cell down and pull on it, but it would be useless info without having a high thrust motor to pull too. Believe it or not, but the boat it's on shouldn't matter as it won't be moving, just pull against the pier until the load cell becomes stable


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Old 25-07-2016, 14:55   #12
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Re: More Prop Diameter or more HP?

Props are tricky, I used to race and all of my props were very expensive and hand made. The larger the diameter the less likely to cavitate. The shape and rake also makes big changes. If you are looking for best performance go with stainless, the pitch on aluminum will flatten out at higher rpms.

Give Jeff Titus a call and I’m sure he will help you out with some knowledge, he is truly an artist with the black art of prop building. He used to build all of my wheels and he makes props for all types of boats.

Tell him Mike Fjeld referred you (that’s me… Fjeld similar to fjord)

Jeff Titus… Performance Propellers 863-465-9438 Florida time
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Old 25-07-2016, 15:37   #13
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Re: More Prop Diameter or more HP?

We've had a few cats with the twin Yamaha 9.9's. Great motors but I don't think they will get you anywhere near the cruising speed you're looking for. What brand and size does MC recommend? We have a single Honda 15 hp on the cat we have now which is 38' and probably weighs a little less than 6,000 lbs. The thrust is unbelievable but cruising speed not quite what I would like, so I'm considering going to a higher pitch prop. You may want to check with Andrew Grace as he has a new Seawind 1160 lite with twin 25hp outboards I believe.


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Old 25-07-2016, 15:40   #14
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Re: More Prop Diameter or more HP?

Here's a YouTube video of a catamaran that uses twin Yamaha 9.9's. He claims cruise speed on both motors of 9 kts. and one motor at 7 kts. which seems close to your target.
https://youtu.be/71i3RBmExGw


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Old 25-07-2016, 15:42   #15
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Re: More Prop Diameter or more HP?

You might look into Kort Nozzles,& other nozzles of differing names, but with the same function, when researching props. Such nozzles increase thrust, especially in low speed high thrust applications. However, I think that props & engines need to be more or less paired with them from the outset.
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