Repitch prop?

[Mike Schmidt]

I have Cummins 330 Diamond 6BTA’s. I phototached them and they’re running at 2750RPM with the boat lightly loaded… around 2700 heavy loaded. My props are 26×24. I verified WOT in neutral at 3150.

Should I assume that 1″ of pitch drop = 200 rpm?

What should I shoot for in terms of heavy loaded WOT? I was thinking 2950 which would require 2.25″ pitch change (from 2700 to 2950 = 250).

Topic

[Tony Athens]

All planning boats should use Class I props or better. But even more important is overall prop design and condition as to whether it is applicable to the vessel needs.. Just like flat tires don’t work with a performance cars, crap props & old fashion designs do not work well on planing boats.

Let’s “see the scan”..

Tony.

[Mike Schmidt]

Tony Athens wrote:

Follow my advise!………

Tony

Yep, the props are getting pulled next week and they’ll be on the way to a very good prop shop. I gave him all the boat/engine specs you advised… and they said they would scan them and then let me know their recommendation.

One interesting option the prop shop brought up is that I may be able to pay a little more to upgrade the prop from class II to a class I. I’m sure they’ll go over the benefits with me after they know more about the props, but do you have any advise on this. Seems that class I could offer a bit more speed and efficiency…

[Tony Athens]

Follow my advise!………

Tony

[Brian Katz]

Apple to Oranges

Mike,

I’m not the expert but I’m currently working on propping my boat as well. One issue is that not all props are created equal. 26×24 prop A is not the same as 26×24 prop B so making adjustments based pitch or diameter is more like trial and error than inquiry based decision making.

A lot of it has to do with DAR or disc area ratio AKA blade area. I’m going from a 22×22 Nakashima to a 21×19 Acme. I was surprised at the recommendation too!

I filled out a questionnaire base on how the boat performs now and what I would like it to perform like. My shop called Acme directly and the two sided put the numbers together.

I was also told if the boat didn’t perform to spec both the prop shop and Acme would stand behind the wheels and make it right. That makes me feel more confident putting all that take home pay down on the counter.

I don’t believe you can be underpropped. I too am worried about losing cruising speed yet I think if you run less prop and can turn a higher rpm it’ll be safe to run the boat at a little higher rpm to offset the speed loss. Maybe someone could solidify that for us.

Cruise can be 85% of WOT so if your WOT is higher you can cruise at a high rpm?????

[Mike Schmidt]

Tony Athens wrote:

Drop your props off at a 1st class prop shop.. He’ll scan them to really see what they are..

Tell him this, and this only..

1) Engines are rated at 315 BHP @ 2925-2950 RPM

2) Tell him the accurate RPM you get now loaded as you use the boat, in gear. He may also ask for a vessel type, weight, and speeds.. Just be accurate in your response

Let him make the decision are to what to do..Personally I would not even suggest what to do. Remember, this is what they do and they should be making the calls.. You are their to provide accurate information, not express opinions. He could even come back and tell you your current props are trash–don’t know.

Tony

Do you think there’s such a thing as being too underpropped. I’m thinking of shooting for the 2950rpm with the boat loaded to the max, but my usual cruising won’t be that loaded… and the WOT rpms would probably be around 3050-3100 range. Is that okay… or should I shoot a little lower?

[Mike Schmidt]

Rob Schepis wrote:

Mike we had a prior topic about WOT no load… did you check it?

I phototached the no load WOT for both engines at 3150, so I’m assuming I can rule out any throttle cable/solenoid issues.

[Mike Schmidt]

Tony Athens wrote:

Drop your props off at a 1st class prop shop.. He’ll scan them to really see what they are..

Tell him this, and this only..

1) Engines are rated at 315 BHP @ 2925-2950 RPM

2) Tell him the accurate RPM you get now loaded as you use the boat, in gear. He may also ask for a vessel type, weight, and speeds.. Just be accurate in your response

Let him make the decision are to what to do..Personally I would not even suggest what to do. Remember, this is what they do and they should be making the calls.. You are their to provide accurate information, not express opinions. He could even come back and tell you your current props are trash–don’t know.

Tony

Makes sense to me. I definitely don’t want to steer the experts in the wrong direction!

[Rob Schepis]

Mike we had a prior topic about WOT no load… did you check it?

[Tony Athens]

Drop your props off at a 1st class prop shop.. He’ll scan them to really see what they are..

Tell him this, and this only..

1) Engines are rated at 315 BHP @ 2925-2950 RPM

2) Tell him the accurate RPM you get now loaded as you use the boat, in gear. He may also ask for a vessel type, weight, and speeds.. Just be accurate in your response

Let him make the decision are to what to do..Personally I would not even suggest what to do. Remember, this is what they do and they should be making the calls.. You are their to provide accurate information, not express opinions. He could even come back and tell you your current props are trash–don’t know.

Tony

[Mike Mason]

In my experience, 1 inch of prop is equal to 100 rpm. Not including cup which can add load as well

[Author]

Replies