Propeller selection assistance needed

[steviethev]

Greetings, I believe that my motor has a propeller that is not well matched for the boat / load. It takes forever to get on plane (and won't with three people on board), has a terrible hole shot, and really bogs down with 2 or more people.

Current prop is a 11.6x13 aluminum of some unknown manufacturer.
WOT RPM is 3900-4000
SOG measured by GPS at WOT (4000rpm) when on plane is 24-25mph
Motor is a 1983 Mercury 50HP 2 stroke 4 cylinder; The manual I have specs 5000-5500 for WOT operating range
Boat is a 1981 Crestliner Nordic 16 (16.16ft) - 800lb dry (Motor weight is about 170; typical people and gear add about 300); total operating weight in the 1200-1300lb range

Based on this, I believe that I should be running a 9 pitch or even lower? Does that seem right for this setup?

 

[SkiDad]

9 seems a little low for that boat setup. Have you verified spark on all the cylinders and have you checked compression ? are you sure that prop is a 13 pitch - maybe it was modified at some point.

if all the above checks out i would say try an 11 pitch b/c I think 9 just doesn't make sense for your size boat.

 

[Sea Rider]

You're 1500 wot rpm off with current load, if that prop has not been modified you will theoretically need to go down 7 less pitches according to Rule of Thumb that states + 200 wot revs per one less pitch. Ideal would be to know how much wot revs are achieved with one less pitch, can go from there maximizing less prop pitches sizes to run at near max 5500 wot revs with current load.

Same 2 engines with same props on 2 different model boats with same lenght size will achieve different wot rpm readings, too many variables to account for. Is engine in good working order throughout the whole throttle range, is engine/transom sitting at the sweet ideal height, is engine correctly trimmed and deck weight evenly distributed ?

Happy Boating

 

[steviethev]

9 seems a little low for that boat setup. Have you verified spark on all the cylinders and have you checked compression ? are you sure that prop is a 13 pitch - maybe it was modified at some point.

if all the above checks out i would say try an 11 pitch b/c I think 9 just doesn't make sense for your size boat.

Spark is great on all 4 cylinders - verified with a tester. Compression was within the manual specification when checked earlier this year, but I will verify again. I agree, going down that low in pitch didn't seem right to me.

This boat is a hand-me-down from my father who replaced the prop last year after wrecking the previous one on there. Apparently what was on there was a 12, but the person who sold him the 13 said 'You won't notice much of a difference', thus I do not think it was modified as it was straight out of the box. I will check the pitch to be sure.

Ever since he bought it, it has been a hog with 2 people riding and struggled to get on plane.

The RPM readings are coming from a tiny tach that I installed recently to understand whether or not it was over-propped. I will verify the installation again, but I don't think it would be off by that much at WOT considering idle was reading right around 850-900.

 

[steviethev]

You're 1500 wot rpm off with current load, if that prop has not been modified you will theoretically need to go down 7 less pitches according to Rule of Thumb that states + 200 wot revs per one less pitch. Ideal would be to know how much wot revs are achieved with one less pitch, can go from there maximizing less prop pitches sizes to run at near max 5500 wot revs with current load.

Same 2 engines with same props on 2 different model boats with same lenght size will achieve different wot rpm readings, too many variables to account for. Is engine in good working order throughout the whole throttle range, is engine/transom sitting at the sweet ideal height, is engine correctly trimmed and deck weight evenly distributed ?

Happy Boating

Everything on the motor is in good working order and smooth throughout the entire range. I will check the engine height tonight and see where it is at. Weight is evenly distributed and the engine is correctly trimmed, at least to the best of my abilities.

This does have a whale tail on it that was mounted a while ago in an effort to help it get on plane. Should I take it off and re-run?

 

[82rude]

If it was my rig I would be trying a 11p solas amita 4 blade.Not sure what it is about solas amitas but I like them a lot .

 

[Sea Rider]

If whale tail is a doel fin and does not ride parallel to water level along anticav plate when combo is on plane will achieve water drag which will slow your wot rpm down a bit. Doel Fins are real garbage collectors.

When on plane while having weight evenly distributed on deck, do you have water splashing at rear of transom or over it, have someone check what's happening at back transom. This cond will also slow engine's rpm down too.

Now imagine boating with doel fin drag along losing top rpm while having back transom water splashes.


Happy Boating

 

[jimmbo]

I would start by making sure the throttle on the engine is opening fully as the throttle lever is advanced. Also check, or get it checked that the ignition is fully advancing. If everything there is fine, weigh the boat, motor, and trailer, then the trailer without the boat. I would suspect you may be packing several hundreds of pounds of water soaked foam under the floor.

As for the Doel-Fin. I would remove it and send it to Davy Jones's locker. Just my opinion

I think an 11 or 12 pitch would in the ballpark on a 16 footer

 

[steviethev]

I would start by making sure the throttle on the engine is opening fully as the throttle lever is advanced. Also check, or get it checked that the ignition is fully advancing. If everything there is fine, weigh the boat, motor, and trailer, then the trailer without the boat. I would suspect you may be packing several hundreds of pounds of water soaked foam under the floor.

As for the Doel-Fin. I would remove it and send it to Davy Jones's locker. Just my opinion

I think an 11 or 12 pitch would in the ballpark on a 16 footer

I will go through the link and sync procedure this weekend to verify that all is working correctly. I do not know the maintenance history of the motor prior to myself so it will be good to have a baseline.

As for the foam, I went through and replaced 95% of the flooring earlier this year and the foam that I discovered was all in good shape. Not to say that there might be some buried that is water logged, but what I could see and feel was not.

 

[steviethev]

If whale tail is a doel fin and does not ride parallel to water level along anticav plate when combo is on plane will achieve water drag which will slow your wot rpm down a bit. Doel Fins are real garbage collectors.

When on plane while having weight evenly distributed on deck, do you have water splashing at rear of transom or over it, have someone check what's happening at back transom. This cond will also slow engine's rpm down too.

Now imagine boating with doel fin drag along losing top rpm while having back transom water splashes.


Happy Boating

There definitely is some water splashing around at the rear of the transom that doesn't look right. I plan on taking off the fin. Also, upon further inspection, it has a bunch of chunks missing and looks bent in an asymmetric way - probably hurting much more than helping.

 

[steviethev]

If it was my rig I would be trying a 11p solas amita 4 blade.Not sure what it is about solas amitas but I like them a lot .

What makes you recommend a 4 blade over a 3 blade?

 

[jimmbo]

Make sure to leave the kids, wife, and mother-in-law onshore, Mistresses and girlfriends are still allowed aboard though