Prop Selection- Now with Tach!

The boat is new to me, and I am just wondering if my speeds sound normal/to be expected. And if a different prop may be better suited to me.

Id like a little more speed, but not sure if i can get it.

Boat:
1983 Coho 134, a 13' whaler clone.

Motor 1982 Mercury 2 stroke 40 HP (i think it has 2:1 lower, best i could find by searching around).

Prop is a Michigan 10 1/4" 14 pitch, Aluminum.

Package should be 6-700 lbs. (I can easily lift one side while on the trailer) I may try to weigh it.

My Wife, 4yo Daughter and I, and our gear, id estimate it about 600lbs total.

Planes and does about 20 MPH WOT, via GPS. I hit 21.2 with the wind and calmer water.
No Tach at this time. No power tilt/trim, the trim rod is in the 3rd hole. It does have dolphin fins on the AC plate. AC plate is nearly dead even with keel.

EDIT: after adding tach, WOT is about 3850 with above load at 21.5 mph.

Re: Prop Selection

Re: Prop Selection

With 40 HP and a not that light whaler clone, probably HP underpowered. What?s the max stated engine for that size sib ? If in pursuit of best prop must install an induction Hardline or TT tach, if hour-tach much better. Go for a wot spin with you alone and a lighted loaded boat and check max rpm achieved with current prop, then will know if over or under propped. 20-22 MPH seems bit low.

Float boat alone, trim engine to be perpendicular to water level, when at plane with a well ballanced boat, boat should ride near paralell to water level, genrally 2-3 hole out transom. Without a tach expect wild answers...

Happy Boating

Re: Prop Selection

Re: Prop Selection

Sea Rider said:

With 40 HP and a not that light whaler clone, probably HP underpowered. What?s the max stated engine for that size sib ? If in pursuit of best prop must install an induction Hardline or TT tach, if hour-tach much better. Go for a wot spin with you alone and a lighted loaded boat and check max rpm achieved with current prop, then will know if over or under propped. 20-22 MPH seems bit low.

Float boat alone, trim engine to be perpendicular to water level, when at plane with a well ballanced boat, boat should ride near paralell to water level, genrally 2-3 hole out transom. Without a tach expect wild answers...

Happy Boating

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I may be over estimating the weight. Im thinking 300 lbs for the hull (whaler states 250 iirc) 250 for motor (again a guess) and 100 for battery and gas.

Max HP is 40. Trim is in 3rd hole now. Feels good on plane but i may try messing with the trim.

Ill have to get a tach it sounds like.

Re: Prop Selection

Re: Prop Selection

83 whaler 13 is 360 lbs motor is about 155, I totaled about 1100 boat motor gas people and gear.
How does the hole shot seem?I think the 15 could be too much. A tach will help tell how things are.
If your experiencing normal slip your rpm is probably pretty low but right now its all conjecture.

Re: Prop Selection

Re: Prop Selection

steelespike said:

83 whaler 13 is 360 lbs motor is about 155, I totaled about 1100 boat motor gas people and gear.
How does the hole shot seem?I think the 15 could be too much. A tach will help tell how things are.
If your experiencing normal slip your rpm is probably pretty low but right now its all conjecture.

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Hole shot is slow, went out today and it was about 10 seconds to plane. I did trim the motor up one more hole gained a mph or two, was able to do 20 with the ladies on the bow. Before was 18.5.

I ordered a mini induction tach. Hopefully that will help with prop advice. I think I was getting some hub slip today.... Spike in rpms in a tight turn, then a couple other times just going WOT in a straight line.

Re: Prop Selection

Re: Prop Selection

I don't think that was hub slip; it is ventilation(cavitation) With your extreme trim setting the prop may be sensitive to turns and other changes that put the prop closer to the surface.
Many times manual trim is not enough in one notch and too much in the next notch. Once we know your performance numbers
A new prop may resist venting better.Some folks tolerate some venting to keep the top speed.
Hole shot will be slower with maximum up trim. A lightly loaded test run will tell better if the setup is working.

Re: Prop Selection

Re: Prop Selection

steelespike said:

I don't think that was hub slip; it is ventilation(cavitation) With your extreme trim setting the prop may be sensitive to turns and other changes that put the prop closer to the surface.
Many times manual trim is not enough in one notch and too much in the next notch. Once we know your performance numbers
A new prop may resist venting better.Some folks tolerate some venting to keep the top speed.
Hole shot will be slower with maximum up trim. A lightly loaded test run will tell better if the setup is working.

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The "slip" i experienced was before i had trimmed up. Sorry i didnt say that. Was in the 3rd hole. I was trying to adjust the trim after running down to the bay to swim and noticing that it appeared the way the engine was sitting would not have the AV plate parallel to the surface of the water. So that's why i tried trimming up when we stopped.

Re: Prop Selection

Re: Prop Selection

If 40 HP is the max rated for, it really doesn't matter how heavy that package is as you can always optimize boat/engine performance with prop pitches specially if there’s a wide range of pitch seizes to dial the best one that matches your boating needs or application. It's a waist of time to constantly wild guessing playing with trim holes without previous tips...

To dial best mechanical trim hole position : float sib, trim engine to be perpendicular to water level (90? angle) place an adult boating mate up front, go a wot spin preferibly on flat no wind water cond, when on plane anticav plate and boat should ride parallel to water level, if in need, adjust trim hole to achieve this condition and leave it there. AC plate water paralelism and leg/transom height issues are different stories. Start with paralelism and follow with leg/transom sweet height match.

Buy a tach hour meter to know engine's cumulative running hours along reading rpm through out all the throttle range, it's worth investing $ 40-50 including shipping on a good quality unit. My boating pal has a 14" footer BW clone with a 40C HP 2 strokes Tohatsu engine, which is the compact 2 cylinder engine with only 500 CC. Used to have very bad top peformance; when took over the corpse, found engine was badly trimmed in, with water splashes over transom and hull was heavily water floded.

Performed a full tune up, raised transom 2 cm, drained all hull water and let it dry for a complete week during hot summer. Went for a spin test, now hole shots near instatly, trim hole performed best at 5th hole out transom, with 2 boaters well deck ballanced is achieving 5600 rpm from max wot stated 5800 rpm. Has gained 250 + revs just by trimming and raising engine accordingly. Next time will GPS it’s wot speed. My pal has passed from an unfriendly smile to a fully wide sideburn to sideburn smile..

Happy Boating

Re: Prop Selection

Re: Prop Selection

Went out today, With a Tach!

Pretty calm, same load as before, left the trim up like last time (4th hole) cruised at 21.5 ish, at WOT and 3850 rpms.

So it looks like id benefit from a different prop.

Re: Prop Selection

Re: Prop Selection

With a WOT rpm range of 4500-5500, I'd say you need a different prop.

The optimum prop for you would be the Solas New Saturn SS 12.25 X 9 to put you right at 5300-5400 rpm and produce the best all around performance.

A 10 pitch (aluminum) would barely get to 5K rpm; where the motor reaches it's peak power.

Re: Prop Selection

Re: Prop Selection

I'm not sure a 14.25" prop would fit on my motor. Seems like a big jump from 10.25"

Edit: went and measured, I only have 6 15/16" from the AV plate to the center of the prop shaft. Depending on what the 14.25" actually measures it may or may not fit.

Re: Prop Selection

Re: Prop Selection

I think he miss spoke I'm sure he intended the Saturn at 11 something inches diameter.

Re: Prop Selection

Re: Prop Selection

steelespike said:

I think he miss spoke I'm sure he intended the Saturn at 11 something inches diameter.

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Would you agree with that recommendation?

Re: Prop Selection- Now with Tach!

First darn typo I ever made! :facepalm:

I meant 12.25 X 9. They also have one in 12 X 9.

Re: Prop Selection- Now with Tach!

jestor68 said:

First darn typo I ever made! :facepalm:

I meant 12.25 X 9. They also have one in 12 X 9.

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That one would fit!

is there a comperable aluminum one that would be suited to me? Im on a budget for this boat.

Re: Prop Selection- Now with Tach!

Sure thing. The prop section has the same size in aluminum listed from Solas, Michigan, and Quicksilver.

Re: Prop Selection- Now with Tach!

jestor68 said:

Sure thing. The prop section has the same size in aluminum listed from Solas, Michigan, and Quicksilver.

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Ok im a prop newbie, why not go with a little higher pitch more more top speed. 14-9 seems like a big jump to me.

Using the prop calculator, Im seeing 23.44 MPH with 0 slip with the 9 pitch.

26.04 with a 10 pitch. both at 5500 RPMs

With my current 14 pitch (and 3850 RPM's), i see 25.52, obviously i dont quite get there, im getting 12-13% slip, why do i need to change other than not running the engine at full RPM's?

School me

Re: Prop Selection- Now with Tach!

The motor is rated 40 hp@ 5000 rpm; with a rpm range of 4500-5500 rpm.

Running below 4500 or above 5500 will cause accelerated wear to the motor. That's why the manufacturer gives that "safe" rpm range for every motor.

You're not getting peak overall performance unless the motor can attain it's hp rated rpm(5000 in this case). So from a performance stand point, the better rpm range is 5000-5500 rpm.

Based on 200 rpm change per inch of pitch, I'll let you do the math and decide how much pitch to reduce in order to get near the optimum 5K rpm. Going a couple mph faster might not be worth it if you're killing your motor by lugging it below 4500 rpm.

Re: Prop Selection- Now with Tach!

java230 said:

Ok im a prop newbie, why not go with a little higher pitch more more top speed. 14-9 seems like a big jump to me.

Using the prop calculator, Im seeing 23.44 MPH with 0 slip with the 9 pitch.

26.04 with a 10 pitch. both at 5500 RPMs

With my current 14 pitch (and 3850 RPM's), i see 25.52, obviously i dont quite get there, im getting 12-13% slip, why do i need to change other than not running the engine at full RPM's?

School me

Click to expand...

Those calculators unfortunately don't take into account the effect of a 1.75 inch increase in diameter with the 9 pitch. Try punching in 11 instead of 9 in the pitch number and see what you get. Also, the slip will be reduced; by how much is the question. Only testing will answer that question.

With the 14, you're showing like 23% slip, which is pretty bad(pontoon territory). The reason is because the prop has too much pitch(angle of attack) and too little shaft speed; so it's stalling and losing thrust. I wouldn't be surprised to eventually see cavitation burn marks on that prop.

Re: Prop Selection- Now with Tach!

It's late, so I'll need to be brief.

1st- you're so far away from the peak HP at 3800, the 200 rpm/inch rule of thumb isn't applicable. You will see much higher RPM's per inch of pitch. You may be making 28HP at 3800 or 33HP at 3800, but certainly not 40HP.

That said, I don't think a prop is your problem at the moment. A 14" prop is very reasonable for that setup and it should easily run 30mph.

I'd start with motor and check compression. If you don't have a tester, you can do a cylinder drop test by checking rpm loss when a plug wire is removed from a cylinder. If needed, just by ear is fine.
Your carbs may need cleaning.
What is the history of the boat?
Since you indicated you can easily lift a side, it doesn't sound water logged.
Just an fyi, a 20hp on an 80's 13' whaler would run 28 and a 40 would run 35+.