Prop Advice Needed: 22' Pontoon with 150HP Mercury

[Dennybot]

Hello! Fairly new pontoon owner here, trying to dial in my prop.

I'll give as much info as possible, and hopefully you gurus out there can point me in the right direction.

The issue I'm having is I am significantly under the suggested top RPM range at WOT.

Specs:

Boat: 2007 22' Sun Tracker Regency
Motor: 1998 Mercury Offshore 150 25" shaft (carbureted) http://boatspecs.iboats.com/Mercury_...8/bpe/57e16405
Prop: Solas Amita 3, Aluminum, 3-blade, D: 14.8, P: 17
WOT: Approx 4500 (speed unknown)
Gear Ratio: Unknown

The above WOT was recorded in calm salt water with approximately 600 pounds of people on board.

We plan on doing mostly lake boating though, and are going for efficient cruising above all else, but it would be nice to get some speed (mid to high 20s) and tow tubes. We will usually have a medium load on the boat (5-8 people).

From the specs I read, the RPM range should be 5000-5600, so I'd say we're aiming for approximately 5400 at WOT. Sound right?

As for the motor, we are confident it is in solid mechanical condition. We have performed compression tests (all normal) and just did a sync&link. It is a 25" shaft motor, which puts the prop a little deep in the water, but I hear that is a good thing. We are assuming the tach is correct, I have to admit, but have no reason to believe it is not (but of course anything is possible).

The boat is in excellent condition and the toons are not holding water.

So...

Is this enough information for any of you to point me in the right direction for the next prop we try?

I'm assuming we need to pitch down. What about diameter? And should I go for a 'work boat' or 'pontoon' specific prop?

Many thanks in advance for your help with this.

Attached is a photo of the transom/lower unit. Perhaps the prop is a little too deep in the water?

 

[steelespike]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

A 16 or 17" prop is typical. But rpm is about 5500 with speeds about 40 a little under/little over.
Can't tell for sure but you may be dragging too much motor through the water.
Probably won't make up 1000 rpm;but you could raise it to a point just before venting becomes a problem
and see what happens.

 

[Dennybot]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

Thanks for the reply, steelspike.

I've given thought to installing jack plates to get the motor a bit higher. But others have steered me toward correcting the problem with prop.

Perhaps it will end up being a solution of both.

Bottom line is I need to make up 700-900 RPM at WOT.

Somehow. If I followed standard formulas, I'd need to pitch down to a 13/14 wouldn't I?

 

[steelespike]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

Yes you can make up rpm with a prop but why would you think when all the tests indicate a 16 or 17" prop running at as high as 5500 rpm a 13 or 14" prop would be right.
I'm not saying the only fault is the motor height but you are dragging 5" more motor through the water than necessary.
It appears that at least part of what is in the water at speed has a square front.

 

[Dennybot]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

I thought pitching down would gain around 200 RPM per inch. I'm at 4500 WOT now, and need to get to around 5200-5300, so that's how I ended up with 13 or 14 pitch.

But it sounds like you're of the opinion I need to get the prop higher in the water.

 

[jestor68]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

You need motor shaft length advice; not prop advice. You put a 25" motor on a pod designed for a 20" motor.

All of the two log pontoon boats at my Tracker dealer have pods measuring 18-19 inches high. That means they use a 20" shaft length, which is what they all have on them.

I can't speak to the tri-toons; having never looked at one. Because they employ a full size center log(26" dia?) they may well use a 25" shaft motor.

I see two options. Either use a "jack plate" to raise the motor, or get the correct shaft length motor.

Does the capacity sticker on that boat really allow up to a 150? I ask because the largest motor that Tracker offers as an option for two log pontoons is a 115/125 class of Mercury.

 

[steelespike]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

I thought pitching down would gain around 200 RPM per inch. I'm at 4500 WOT now, and need to get to around 5200-5300, so that's how I ended up with 13 or 14 pitch.

But it sounds like you're of the opinion I need to get the prop higher in the water.

Your right you can increase rpm by reducing pitch but::
Besides the fact that the typical; 150 hp 2 log pontoon prop is 16 or 17" .
A 17" prop with 20% slip would do about 38 at 5500
A 14" prop with 20% slip would do about 31 at 5500.
20% slip being probably the minimum slip on a 2 log pontoon.

 

[Dennybot]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

I bought the boat used and did not know the motor length was incorrect... until recently.

Good question on the motor HP. From what research I have done (phone calls and boat forums) we are at the upper limit for motor HP and weight. The motor actually exceeds the weight limit by 10 pounds.

So, indeed, it is a matter of determining whether I should try to make this motor work, or consider the costly alternative of replacing the motor with something more suitable.

We did a measurement of the cavitation plates to the bottom of the pod, and it's right around 6" too low.

I found this simple (and cheap) Jack plate, which says it'll give up to 5.5" of lift.

I wonder if this would do the trick?

http://www.thmarine.com/products/Ou...atic-Height-Plates/Hi-Jacker-Fixed-Jack-Plate

 

[jimmbo]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

Looking at the suntracker website. The 22 regency list of motors do show extra long shaft engines or the offshore in the case of the115(i will assume 25 inch is the standard for offshore models) it also shows 150 as the max hp

 

[Dennybot]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

jimmbo, can you tell me where you found that info?

Because that would counter what a dealership told me just this morning, that all late-model Tracker pontoon boats are designed for a 20" shaft.

BUT, that same dealer suggested I simply pitch down to get into the right RPM range.

Ideally I could resolve this problem with a prop. But I don't want to do that at the expense of decent performance and longevity of the motor.

 

[jimmbo]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

http://www.suntrackerboats.com/boat/motors.cfm?boat=3314

I did make a mistake regarding the 115 as being an offshore, the list is just 4stroke 115. the smaller engines sre ELPTmodels - Extra long Power trim


Contact the manufacturer and ask them
http://www.suntrackerboats.com/about/contact.cfm

 

[steelespike]

Re: Prop Advice Needed: 22' Pontoon with 150HP Mercury

You could run the boat and note if the motor seems too deep at speed.
I would think only the streamlined portion of the leg should be in the water.
Not saying it is the only performance problem but could surely be part of it.