# Prop size help

Discussion in 'Props' started by SwedenYachtsDaniel, May 6, 2021.

1. Joined: May 2018
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### SwedenYachtsDanielJunior Member

Hi.

Looking for advice on prop size for my sailboat. I have asked around getting prices and suggestions from different places and i get very different answers
Im looking for a 2 blade folding prop.

Her is the numbers:
Sweden Yachts 38 sailboat.
LOA 38 feet
Lwl 31 feet
Displacement 9 ton.
36 hp Lombardini Ldw 1404m
3600 rpm
Lombardini saildrive 2,18:1

The three different prop suggestions goes from 17x11 to 13x10 !!

The prop i have today is unmarked and to big. The boat goes at 7 knot at 2800 rpm.

Any suggestions is appreciated

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### missinginactionSenior Member

Theoretical displacement hull speed is calculated by the formula: velocity in knots = 1.35 x the square root of the waterline length in feet.
The square root of 31 is approximately 5.57. 5.57x 1.34 = 7.46 knots. That's pretty close to what you are getting with your existing rig. You didn't give a beam figure but if she's a little beamy that will slow you down a bit.
So given that she's pretty close to her theoretical displacement hull speed, what are you trying to accomplish?

MIA

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### jehardimanSenior Member

Question? Why do you think it is "too" big?

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### fastwaveSenior Member

A prop can never be too big. Diameter gives you efficiency. The trade off is when sailing especially if not a folding prop.
What you mean the pitch is too high.
If the engine max rev is 3600 and you can only achieve 2800 at full throttle then you are way off.
An engine should be able to achieve it’s max revs and even a bit more. This is too allow for rough weather and some fouling.
As it is you are running the engine at higher torque which leads excess heat load on the engine.
What the right pitch is depends on your prop diameter. But if you stay with your current diameter you definitely need a pitch reduction

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5. Joined: Jan 2010
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Impossible to reach a speed of 7 knots with LWL=31 ft , Displ. 9t and propulsion plant 36hp/3600 through gear ratio 2.18.
The propeller is not so big but so heavy for 36 hp / 3600rpm / r=2.18
Choose 17"x 8" and you will run at 6+knots at full engine speed (3600rpm).

Last edited: May 7, 2021
6. Joined: Aug 2007
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### missinginactionSenior Member

Alder, the OP is in fact reaching displacement hull speed with well under 36 horsepower. It takes very little power to reach displacement speed. I do it with a small planing cruiser (LWL 21 feet, beam 10.5 feet, displacement 3.5 tons). For me it only takes about 2/3 throttle with a Yamaha T 9.9 high thrust outboard kicker.

There are a couple of misconceptions here. Fastwave is telling the OP he is "way off". I don't have every piece of relevant data but based on what we know I'd argue that the boat is powered and propped properly for it's size. Alder isn't going to go faster than displacement speed. It's a sailboat. It's a full displacement craft. If 2/3 throttle on the current engine and prop gets Alder to displacement speed then just run at that throttle setting. Going to full throttle will gain you nothing in terms of speed (or very, very little) and will just waste fuel. Putting a new prop on this boat isn't going to gain you any speed it will simply allow the engine to spin faster. This will be a little easier on the little diesel but materially no different than throttling back to 2800 or so with the existing set up. No need to reinvent the wheel. To quote an old mechanic friend of mine: "If it ain't broke, don't fix it.".

7. Joined: Jan 2010
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He needs more power to keep the existing prop and to reach the hull speed ; for that engine 36 hp Lombardini Ldw 1404m this propeller is too heavy. Is better to change propeller at 17"x 8" than to keep the engine suffering.
Another solution is to change the gearbox ratio at 2.7. In any case the existing propulsion plant is not balanced. As you know well when will comes the time to sell the boat no one buying a boat with an engine which can't reach the top speed.

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Last edited: May 7, 2021
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### gonzoSenior Member

Obviously not true since the OP bought it.

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### fastwaveSenior Member

it has nothing to do about achieving better speed. If the engine cannot rev more than 2800 it is because it is reaching it max torque limit at that rpm. It running at its limit. That is very bad for engine life and fuel consumption is not great either. An engine running at 2800 rpm does not always burn the same fuel. It’s load dependent.
This would be the same for other rpm. You are forcing the engine to work near it maximum load without any benefit. A proper propeller needs to take into account the engine torque curve with rpm and the torque characteristics of the prop at different rpm. For most engines the torque curve characteristics are such that you will never exceed the torque at intermediate rpms and can focus on dimensioning the prop at the max case.
However it is possible that a certain engine and prop combination will not reach max rpm even though the engine has enough torque for this single point, because the propeller torque Exceeds engine torque in medium rpm. This means the prop will never turn faster.
Like said It’s not only about boat speed

10. Joined: May 2018
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### SwedenYachtsDanielJunior Member

Thanks for all suggestions so far.

FYI: I bought the boat whit "engine" problem to a reduced price. I quickly found out that the engine was fine and prop was way too big (3 blader 17x12) . So i got my hands on a used smaller prop that is unmarked , or the markings cant be seen, but is said to be 15x11. That prop is really worn and need to be replaced. Whit this prop my boat reaches 7 knot at 2800rpm at full throttle . At this rpm im missing the last 5 hp. So this prop is too big.
So i asked Gori and the suggest 2 balded 16,5x11 , i think that is way to big, don't know how they got that answer
Flexofold first suggested 17x11 at first. But when i asked how they got to that they said it was by looking at a list they had whit a different gear ratio.
An other Flexofold dealer suggested 13x10, that sound a bit small to me .
Vicprop calculator suggest 15,8x8,1
Im thinking 15x10 would be a good fit. Maybe 17x8 as Adler suggested is good but its harder to find then 15x10.

So the reason i ask is to get a propeller that let the engine reach 3600 or very close to, so i get the full potential of my engine and not overload it.

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### jehardimanSenior Member

Ok, first of all propeller diameter can never be "too big" (within physical reason) unless you are having cavitation issues, which we will check for the prop size you stated and the engine/gearset you have.
More likely, you ran out of torque due to prop pitch and/or blade area ratio (BAR). Because you don't know the actual pitch and BAR of the supposed 15x11 (I assume 3 bladed 0.5 BAR) you still are absorbing too much torque.
As MIA pointed out, you are not going to get too much more speed out of that last 5 shp.
If we assume that you actually have a 3 bladed 15x11 0.50 BAR, then your existing slip is ~166% and your ehp is ~22 hp, which means the prop is not doing that bad. Local cavitation number is ~0.61 and the thrust loading coefficient is 0.475 meaning you are getting ~30% back cavitation. Which means for the existing thrust and blade area we need to increase the prop diameter and/or reduce the pitch. Conversely, if we hold diameter and thrust we will need to reduce BAR and pitch.
From the original 17x12 3 bladed, I'd start with pitching it down and removing blade area. Gori's suggested 2 bladed 16.5x11 is reasonable if the BAR is greatly reduced. I know Flexofold props have low BAR for their size so a 17x11 two bladed is again reasonable. I'd be interested in the vicprop calculator to see if that was a 2 or 3 blade prop because the pitch seems too low. I'm not sure I'd take the pitch as low as Alder would like, but I'm sure I'd keep the diameter. Additionally, Alder's comment to raise the gear ratio has merit if you want to keep the BAR up for maneuvering reasons.

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12. Joined: Jan 2010
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Dear All,

The referred boat class ~ Sweden Yachts 38 sailboat ~ was equiped in standard mode by Volvo type 2003TS / 45hp at 3200rpm and a Reduction Gear Drive with Ratio 1 : 2.2.
A 2-blades propeller sized to 18"x 10" at top engine's rpm pushed the Boat at a max. speed of 7+ kts. A well known sense that defines the basis of cruise speed determination of a boat
is the amount of power that the propulsion engine delivers at a speed Area where the Crankshaft applies the highest amount of Torque. On that Point the Specific Fuel Consumption factor
reaches the Lowest rate. In case of Volvo that point is determined at 2750 rpm and the corresponding delivered Power reached to 29 hp max. This amount of Power through the
Propeller could be produced thrust to push the boat at a speed of 5.9+ kts. This is the cruise speed of this boat class with Volvo engine.

In our case with Daniel's Yacht 38 we faced a mismatching of Installed Propulsion Engine's Power at 36 hp (9 hp less than Volvo) to the Propeller's size that remained the same of 18"x 10".
The engine's speed due to trials was blocked by fuel pump governor of 36 hp Lombardini Ldw 1404m at 2800 rpm and the boat speed of course was not 7 kts ~as referred~ but actually 6.3+ kts .
The deviation of +0.7 kts may be was an influented result relevant to stream , waves and wind due to sea-trials. Finally a 2-blades propeller's size suitable to the specific hull and present installed Power
propulsion plant should be near to a propeller size of 17"x 8" to prevent wear and tears.

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13. Joined: May 2018
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Location: Norway

### SwedenYachtsDanielJunior Member

When i say to big i mean over-propped, the engine cant on this boat reach the rpm it is suppose to ..

I did get hold of a old picture of my now existing prop and it says 15 x 11 LHS. 50381

I found my notes from my propeller and top rpm test i did, i reach was 2700, not 2800 as i have written her before. Plus i did it in two directions and the speed was 7 knot and 5,4 knot. So average og 6,2 knots , close to what Adler is saying.

The original engine is a 28 hp Volvo Penta , not 45 hp

At 2700/2800 rpm my now existing engine does not produce 36 hp but about 31 hp

the Vicprp calculator suggest: 2 balde 15,8z8,1 or 3 blade 15,1 x 8,0 and as a rule of thumb that one inch of diameter is equal to 1,5-2 inch of pich.

So now i know I am over propped whit 15x11 2 blade.

I guess a 17x8 might work but it seams to me that the are nor so easy to find as a lets say 15x10

What rpm Do you think i reach if i go from 15x11 to 15x10

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