Prop Pitch/size

[DrWelsh]

I am a relatively new owner of a Mainship 430 with twin Cat 3116 engines (350 hp each). I am trying to: 1) Determine the best size props for the boat and 2) what the pitch of the props should be.

John Welsh

 

[Ski in NC]

Usually the boat MFR gets pretty close to optimal props when boat is delivered. What props do you have now and what makes you think they are not right?

It takes much more data than give to select optimal props.

 

[SoWhat]

Prop calculator at:
https://www.vicprop.com/displacement_size.php

May not have any correlation to what is actually installed on your boat. It is simply a starting point. If you can hit max rpm youre in the ball park already.

 

[Ken E.]

As Sowhat said, if you can easily come up to max rated rpm, you're in good shape. Since the boat is new to you, make sure you've got max weight in it when you run it up....full fuel and water, beer, wife's clothes.....

 

[ben2go]

Prop calculator at:
https://www.vicprop.com/displacement_size.php

May not have any correlation to what is actually installed on your boat. It is simply a starting point. If you can hit max rpm youre in the ball park already.

That prop calc can be pretty accurate if you know the beam at waterline and waterline length, actual weight of the boat, and hp. Travel lifts usually have a scale built in that can give a pretty accurate weight.

 

[DrWelsh]

Thanks for all the input.

I just measured the props and they are 20". The pitch seems to be stamped on the shaft of the prop.

I don't know if these are the original props. I can hit 2850 rpm but the most speed I have gotten is 13 knots. Getting a little vibration on the starboard side. Cutless is in good shape.

I am having the starboard Cat 3116 rebuilt. while the boat is pulled I want address any issues that require the boat to be out of the water.

 

[DavidM]

Thanks for all the input.

I just measured the props and they are 20". The pitch seems to be stamped on the shaft of the prop.

I don't know if these are the original props. I can hit 2850 rpm but the most speed I have gotten is 13 knots. Getting a little vibration on the starboard side. Cutless is in good shape.

I am having the starboard Cat 3116 rebuilt. while the boat is pulled I want address any issues that require the boat to be out of the water.

The Cat 3116 is rated for 350 hp at 2,800 rpm. If your reported rpms of 2850 are correct, that is about where you want to be and no change in prop is indicated. But dash tachs are notoriously inaccurate and a $25 phototach will tell you exactly what rpm you are running.

Vibration is another issue and could be caused by a dirty bottom, misaligned engine, worn cutlass bearing, bent prop or prop shaft, prop not seated right on the shaft, coupler not square to shaft, etc. You might want to check the engine alignment first while the boat is in the water and see if aligning helps. Otherwise all of the others can be addressed once the boat is out of the water.

If you confirm the rpm readings are ok (and the bottom is clean), then you might want to pull the props and get them tuned to Class 1 specs. If the real rpms are near or below 2,800 then a prop shop can repitch them for you to get to at least 2,850.

Unfortunately 13 kts is probably all you are going to get with those engines which probably means a max cruise of 10 kts. And I wouldn't cruise at that speed because the boat is probably pushing a big bow wave and it will really load the engine. Drop down to 8 kts and enjoy trawler speeds.

And finally why is the engine being rebuilt? Could it have been overloaded as above. That might mean that the port engine is next.

David

 

[Captains Quest]

The 3116 engine was originally build with a soft block that deteriorates and has to have a sleeve inserted to correct. Initially catapillar corrected the problem but only if you asked but it appears the earlier owner didn’t fix it.

 

[Shrew]

Diameter is a relatively fixed value. The clearance of the Propellor (distance between prop blade tip to hull)should be 15% - 20% of diameter. (Ideally 20% or more).

Once you have the diameter, the Pitch is calculated, by and large, as a result of Max RPM. Theoretically, if you can exceed the max rpm, your underpropped (pitch needs to be increased). IF you cannot reach max rpm, you are overpropped (pitch needs to be decreased).

This is a loose rule of thumb to which there are many opinions. Tuning is somewhat of a dark art IMHO.

 

[High Wire]

If you are taking one prop in you should bring the other to match. Write down the transmission ratios off the nameplate of each trans. They may be slightly different and the prop shop will want to know.

 

[Martin J]

Something does not seem right. Read this review and performance article.
https://dicksimonyachts.com/Brochures/Mainship/430review.pdf
This review is for 2 x 350hp

Also I would suggest you contact Michagan Wheel they have a huge library of information for just about every boat As a company they are extremely helpful.

 

[vacser]

John, I owned a 2003 43 Mainship Aft Cabin with 370 HP Yanmars. Came from factory with 4 blade 25" Diameter wheels. Pitch was changed over the years to end up at 25" also. Yanmars turned 3350 WOT to run 19.5 knots with a cruise at 2900 RPM of 15 Kts. Hope this helps

 

[High Wire]

20 inch diameter prop does not sound right on that boat

 

[rslifkin]

Diameter is a relatively fixed value. The clearance of the Propellor (distance between prop blade tip to hull)should be 15% - 20% of diameter. (Ideally 20% or more).

Once you have the diameter, the Pitch is calculated, by and large, as a result of Max RPM. Theoretically, if you can exceed the max rpm, your underpropped (pitch needs to be increased). IF you cannot reach max rpm, you are overpropped (pitch needs to be decreased).

This is a loose rule of thumb to which there are many opinions. Tuning is somewhat of a dark art IMHO.

Diameter is somewhat fixed in practice, but more variable in theory. Unfortunately, most of us don't have the clearance to run any diameter within the reasonable range to see what works best, so we're somewhat limited to just run the biggest prop that fits.

I'll also agree with the comment that 20" diameter seems too small for a boat as heavy as the 430 with that amount of power. The 25" diameter props mentioned by vacser seem much more reasonable.

If we knew trans ratio and prop pitch in addition to what's been posted here, we could figure out how much slip there is to see if it's excessive. If it is, larger diameter props would most likely help performance.

 

[porman]

The props on our 430, powered by two Yanmar 370hp engines, are 25 by 24.