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Old 25-07-2016, 14:46   #16
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Re: More Prop Diameter or more HP?

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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
Do you know what pitch you have on your prop. Best I can tell, the standard is a 9.25" x 10" pitch 3 blade. So a 10"x7" 4 blade should pull better. But you point out the salient question. What is the load, rpm, slip ratio etc that causes a particular prop to loose grip.

Since the boat is designed to have the prop center 14" under the surface of the water, I have thought about sawing off the anti-ventilation plate and putting a 12" prop on there. It's so tempting. Wish I could find someone who has done it.
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Old 25-07-2016, 15:11   #17
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Re: More Prop Diameter or more HP?

Mine came with a 10" pitch, I think that is standard for the short shaft, I do not know about long shaft but concede the logic that it would be less. I have a Solas 11" SS prop and a 12" Solas SS prop, the 12" is just too much and I am sure there is more pitch on a similar number Solas SS prop compared to the stock aluminum Zuk prop, I believe it is in the amount of cup there is on the trailing edge. I bought the 12" and it will run right at 35 kts once it crawls onto plane, but two people and it struggles and crawls with everyone leaning forward to nurse it on plane, just over propped so I dropped back to an 11", I think A 10" in SS would be the ticket, but I've spent too much on dinghy props as it is.
I'd think as many blades as you can get with as little pitch that you can get would work best. As far as sawing off the plate, you mean I guess the whole thing? Never heard of that being done, surely it has to be structural, much as they were trying to reduce weight, I can't imagine it's not needed for structure.
I don't know if you could fit a Kort nozzle but that would help some, a Kort nozzle is similar in effect to the winglet on an airplane in that it interferes with and breaks up the wing tip vortices which increase drag.
Look at almost all scuba scooters to see a Kort nozzle, they are great prop guards too.

It may work for you with a four blade, I do not think it will be what you want with a three blade, at about 3/4 throttle it will cavitate and my motor is set a little deeper than it should be, so its not ventilation I'm getting, it's cavitation.


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Old 25-07-2016, 15:13   #18
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Re: More Prop Diameter or more HP?

OK rereading your post your correct I was thinking backwards, yes it came with a 9.25" diameter prop with a 10" pitch.
Can it handle a 10" diameter prop? If so even that 3/4" would I think make a difference


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Old 25-07-2016, 15:49   #19
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Re: More Prop Diameter or more HP?

I just noticed that out of the 3 outboards your interested in only one comes with a 25" shaft, which to me has great importance. I'm pretty sure you can get a 5" extension kit for the Yamaha 25 but unsure about the Suzuki.


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Old 25-07-2016, 15:56   #20
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Re: More Prop Diameter or more HP?

Somebody tell me what is wrong with the high thrust Mercury 15?
Does it not fit the bill, I'm sure it will easily pull my 20 HP Zuk backwards at a good clip, and is much lighter than the 25 Yamaha.


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Old 25-07-2016, 16:01   #21
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Re: More Prop Diameter or more HP?

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Somebody tell me what is wrong with the high thrust Mercury 15?
Does it not fit the bill, I'm sure it will easily pull my 20 HP Zuk backwards at a good clip, and is much lighter than the 25 Yamaha.


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Only problem I see is it doesn't come in a 25" shaft. The Honda 15 comes in a 25" shaft and is 20 lbs lighter than the Mercury 15 in a 20" shaft.


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Old 25-07-2016, 16:29   #22
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Re: More Prop Diameter or more HP?

You might try posting this query on BoatDesign.net if you haven’t already. Some of those guys are pretty sharp on this stuff. And I've seen discussions on this exact topic over there before, as well as bits on Kort Nozzles, etc., so it couldn’t hurt.

Also, a few months back, there was a 35’ Shuttleworth cat for sale. She had designer approved transom extensions with a pair of large outboards on them, possibly 25hp’s. And while she’s since sold, IIRC one of the places which she was listed was Shuttleworth’s site. So between that, & his having given the approval on adding bigger scoops to her, odds are he’d know some of the particulars about the engines which she has/had. In addition to knowing how to reach the owner who added them, & would know all of their particulars. So if you can get a POC from his office…


PS: The boat listing might also be findable via the Google Cache? Though I'm not up to speed on using such.

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Old 25-07-2016, 17:02   #23
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Re: More Prop Diameter or more HP?

Not sure if this is the Shuttleworth you are talking about.
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If so, about 10 years ago she was undergoing a conversion to diesels. The owner at the time said she cavitated badly with the 50 hp outboards. A local from Miami bought her, completed the diesel conversion and also added an additional 2' to the sterns to most probably compensate for the weight of the diesel. Since then, about 3 years ago she sold to friends of ours and resides in the NE of the US. A really pretty boat, but probably the worst place to mount outboards in my opinion.


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Old 25-07-2016, 19:54   #24
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Re: More Prop Diameter or more HP?

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Originally Posted by a64pilot View Post
Somebody tell me what is wrong with the high thrust Mercury 15?
Does it not fit the bill, I'm sure it will easily pull my 20 HP Zuk backwards at a good clip, and is much lighter than the 25 Yamaha.
Nothing wrong with it, other than a bit heavier than the competition. Last I talked to the guy who was getting hull #2, he was going w/ the Honda 15. I'm hull #3. I'll certainly learn from him, but still missing the underlying theory of operation. Just trying to understand the variables of Prop size, gear ratio, HP, etc to I can make the right choice - can't really try all the combinations of motors/props. I lean to the Suzuki since it's EFI and reportedly doesn't have the carb challenges of the competition. The computer is also supposed to do better at fuel economy on a long cruise. But the reliability of the EFI is the bigger issue for me.

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I just noticed that out of the 3 outboards your interested in only one comes with a 25" shaft, which to me has great importance. I'm pretty sure you can get a 5" extension kit for the Yamaha 25 but unsure about the Suzuki.
Yes, Bay Mfg makes a 5" kit for the T25. For the suzuki, while I didn't use a microscope, I spent a couple hours going page by page through the parts list/diagrams, and the only difference I could find between the 25" shaft DF9.9btx and the DF20a is the air intake restrictor plate and the computer. Parts cost about $350/engine. A lot cheaper than buying the df20a and the leg for the 9.9 and putting a long leg on the 20.

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You might try posting this query on BoatDesign.net if you haven’t already.
Thanks, done.

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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.
Thanks, will do.


The local dealer said Suzuki doesn't have a consumer help line, dealers are supposed to take care of that. Suzuki America is in So Cal, haven't tried cold calling them yet to see if I could find someone knowledgeable.
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Old 25-07-2016, 20:32   #25
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Re: More Prop Diameter or more HP?

Congratulations on your new Maine Cat, great boats and the 38 looks like it is going to be exciting to sail. If I had to make the choice your forced to make I would be going back and forth between the Yamaha 9.9 and the Honda 15. Both come in the 25" shaft and there weights are close to equal. The Honda in my opinion has the advantage of being a larger motor, 350cc's verses the Yamaha 9.9's 212cc. But then again the Yamaha's lower unit has a lower gear reduction. Any chance you can get feedback from the owners of hull #2 with the Honda 15's before making a decision?


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Old 26-07-2016, 06:58   #26
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Re: More Prop Diameter or more HP?

I can't help but wonder if Richard Woods the multihull designer/builder/sailor, would have some expert insight into this issue. As many of his boat designs use outboards, & he's also a member here. So he weighs in on multihull questions from time to time.

Not to add more to your array of choices, but not long ago there was a cat of about the same size as yours listed for sale, which had a diesel in one hull, & later on, an outboard in a sled was added next to the opposite hull. The outboards purpose being to aid in manuvering the boat in harbor & when docking.
The builder, Green Marine, had decided with the owner, against 2 diesels in order to save weight. But they found that the boat needed 2 engines for safer handling.

Anyway, I'd be curious to know the bulk of what's discovered, information wise, about props & engines. And what setup you decide on. As this seems a fairly common issue faced by mid-sized & small multihull (& some monohull) owners.

smj, Yes, I think that your posted picture of a Shuttleworth 35' is the boat which I'd mentioned. Nice design, but for lacking a bridgedeck cabin. But if someone gave her to me I'd muddle through
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Old 26-07-2016, 07:07   #27
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Re: More Prop Diameter or more HP?

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I can't help but wonder if Richard Woods the multihull designer/builder/sailor, would be able to offer some real insight into this. As many of his boat designs use outboards, & he's a member here. So he weighs in on some multihull questions from time to time.

smj, Yes, I think that your posted picture of a Shuttleworth 35' is the boat which I'd mentioned. Nice design, but for lacking a bridgedeck cabin. But if someone gave her to me I'd muddle through
Great minds think a like. I have an email into Richard and Bob Gleason (been handling corsairs and seawinds for a while).

smj, yes plan to learn a lot from #2. Perhaps I'm being too analytical, but seems like there should be some formulas that quantify the tradeoffs.
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Old 26-07-2016, 07:19   #28
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Re: More Prop Diameter or more HP?

If the Honda 15 is a high thrust long leg, big prop and lighter weight, go that route. Nothing wrong with a Honda, and carbs are I believe a solved problem, stay away from Ethanol fuels and use marine Sta-Bil, run the carbs dry after each use.
Do not try to make a Science project out of this and go about modifying a 20 HP Suzuki or trying to make a 20 HP out of the 9.9. I'm pretty sure the ECU is more expensive than you might think, many manufacturers use the same engine block with only ECU changes to determine HP. Mercury Verado's whole line from 150 to 350, now I guess 400 HP is only two engines with different ECU's, and they price the ECU's so that it's more expensive to buy a higher HP ECU than the higher HP engine.
I'd be surprised if Suzuki doesn't do the same.
I am certain that in the long run staying with a factory unmodified product will yield less headache
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Old 26-07-2016, 08:57   #29
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Re: More Prop Diameter or more HP?

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Great minds think a like. I have an email into Richard and Bob Gleason (been handling corsairs and seawinds for a while).

smj, yes plan to learn a lot from #2. Perhaps I'm being too analytical, but seems like there should be some formulas that quantify the tradeoffs.
Good to hear the above. And this is so common sense that I didn't even mention it before, but you've posted this question on some of the other sailing forums, non? Especially the purely Multihull ones.
Also, what's your dreadline, er, deadline for buying these power plants? If I may ask.
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Old 26-07-2016, 09:03   #30
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Re: More Prop Diameter or more HP?

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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
Unless weight and price are a real concern, you may want to consider a pair of Honda four strokes with the 28" leg. I know they make it in 15hp they may make it in 9.9? Probably the same engine. I think you could achieve hull speed very economically. Over hull speed I don't even think about. It is just a waste of fuel.
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