Speeds to expect on modified tin boat?

[Taxin_That_Bass]

Hey guys, just joined the site, love the fact that its all about tin boats. First off i'm going to pre-apologize about the length of this post.
Quick background on myself, bass fisherman from La Crosse WI, have always owned a bass boat for tournaments and a tin boat for fun fishing in the sloughs. Sold my bassboat last year as well as my 14 monark. Got a steal on a 1972 Polarcraft 16ft flat bottom with coast guard nose with a 1981 evinrude 35hp. I wanted to convert it into my "tin bass boat", i knew going into this that i would add significant weight and would lose speed. So...

i added a large deck up front that runs from the front of the boat and covers the front bench seat, with recessed trolling motor pedal, tackle compartment that stores a tote with tackle, life jackets ect. Also added side lockers for storage that run between the middle and front bench seat that store one grp 31 battery, noco on board charger, anchor and other misc items. I added a back deck that covers from the transom over the back bench. Also added floor between back and middle bench seat and middle and front bench seat. Transom has 6 gal gas tank and another grp 31 battery. I have a 70lb maxxum bow mount trolling motor up front aswell. All of the decking, floor, and framing is made from plywood and 2x4. Obviously this is much heavier than aluminum, I considered using aluminum but at the time it was just way too expensive (may to re do it at some point).

Anyway, so i have added a quite a bit of weight to the boat. I also added a tachometer to the motor, by myself i am getting about 24 mph gps at about 4500 rpm wot with semi beat up 10x13 aluminum prop (edges on two blades are chewed a bit). Two people drops to about 20 or 21 and dogs to get on plane.

Questions: 1) 24mph on a heavily loaded 16 tin boat seem appropriate?
2) Do you think a SS prop will help me much?
3) do you think dropping to something like 10x11 to get my RPM closer to 5500 will help much on top end speed? I would think it would help with hole shot at least

Thanks in advance.

 

[Johnny]

Welcome Aboard !!

I guess I am too Old School - - - but, why isn't 24 miles an hour enough ????



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[Taxin_That_Bass]

Welcome Aboard !!

I guess I am too Old School - - - but, why isn't 24 miles an hour enough ????



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Haha good question, 2 part answer: 1) I fish the upper mississippi river and it's giant, a few more mph won't make much of a difference getting from point A to point B but it my mind it does
2) I just feel like 4500 rpm just isn't getting to the peak point in the HP curve for my motor, so i feel like i'm leaving some HP/speed on the table. If a different prop will help it out, then great, if not than i can put it behind me and go with what i have.

 

[Johnny]

our fellow Motor Guru, Pappy is the local expert in the
1980s OMC outboards . . . . he will give you the correct specs.

if you are looking for speed and power - I would try to budget for a 50hp Johnson and be done with it.

my grandmother had a saying . . . "you are wanting too much sugar for that dime"



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[DaleH]

It is not MPH you should be concerned with, but RPMs!

IMHO you are severely lugging that motor and besides not adding oil, using the wrong prop is a sure way to kill a motor.

By my search that is a 5800 RPMs maximum motor and if it were mine, I'd never settle on a prop until I reached at least 5600 when running the boat alone, 1/2 fuel and with all gear typically carried.

 

[Stumpalump]

What DaleH said.. Skip the stainless.

 

[Stumpalump]

What is a "Coast Gaurd Nose" ???

 

[Weldorthemagnificent]

If nothing else a 10x11 would get you out of the hole easier. I'd stick with aluminum.


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[Crazyboat]

I think the rough rule is you gain 200 rpm for each inch of pitch (I may be very wrong). You need to increase your RPM to the max recommended for the outboard which I think is 5,500 with a light load. As you add a fishing buddy your RPM will drop and that's OK.

24 MPH sounds about right and I doubt you'll get much more no matter which prop you go to, if you hit 26 be happy, unless you are willing to drop the weight for another 1-2 MPH.

 

[Taxin_That_Bass]

It is not MPH you should be concerned with, but RPMs!

IMHO you are severely lugging that motor and besides not adding oil, using the wrong prop is a sure way to kill a motor.

By my search that is a 5800 RPMs maximum motor and if it were mine, I'd never settle on a prop until I reached at least 5600 when running the boat alone, 1/2 fuel and with all gear typically carried.

i was a little concerned about that. Can you tell me why it is hard on the motor? Why operating at that point kills motors?

 

[Taxin_That_Bass]

Also, can you recommend a prop for my setup then? Was looking at the solas 11x9. I think i need a significant drop in pitch to get my rpm where it needs to be

 

[Stumpalump]

Post pic of your original beat up prop.

 

[edlacy]

Also, can you recommend a prop for my setup then? Was looking at the solas 11x9. I think i need a significant drop in pitch to get my rpm where it needs to be

You're lugging it.

You wouldn't drive a manual transmission car 15 MPH in high gear. You're doing the same to your outboard.
As was stated above, you want 5300-5400 RPM @ WOT or you'll never know the full potential of you motor. As well, you're shortening the life of the motor. Those Evinrude/Johnson motors are real work horses, but any motor can only take so much abuse. Running your motor at low RPM is abuse.

 

[LDUBS]

I think the rough rule is you gain 200 rpm for each inch of pitch (I may be very wrong). You need to increase your RPM to the max recommended for the outboard which I think is 5,500 with a light load. As you add a fishing buddy your RPM will drop and that's OK.

24 MPH sounds about right and I doubt you'll get much more no matter which prop you go to, if you hit 26 be happy, unless you are willing to drop the weight for another 1-2 MPH.

Aw shucks, never mind. I should have read the whole thread -- :|

 

[Taxin_That_Bass]

Post pic of your original beat up prop.

I will try to get a pic of it tonight. I hate buying props without trying them, but i might order a 11x9 solas and see what that does.

I'm pretty confident my rpm measurement is correct, but just to confirm, i got one of those induction tachs where you wrap it around the spark plug wire. There are three settings on it, 1 pulse per 1 rev, 2 pulse per 1 rev, 1 pulse per two revs. I at first set it to one pulse per one rev but it was obviously way too high, i changes to 2 pulse per 1 rev and then the numbers looked more believable. I guess some of these older outboards will spark when the piston is at bottom dead center as well as TDC?

 

[dirty dave]

Where did you order tachometer from? And how much. Im interested in adding one to my boat.

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[Taxin_That_Bass]

Where did you order tachometer from? And how much. Im interested in adding one to my boat.

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https://www.amazon.com/Hardline-Pr...&sr=8-2&keywords=hardline+products+tachometer

 

[ktoelke54]

In my experience you'll need to only drop to 12 on pitch possibly 11 to get where you want on rpm. If you go from 13 to 9 your top speed will be less than 20 and you'll be over revving.


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[Taxin_That_Bass]

In my experience you'll need to only drop to 12 on pitch possibly 11 to get where you want on rpm. If you go from 13 to 9 your top speed will be less than 20 and you'll be over revving.


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Why experience tells you that? I have always heard 200 rpm per inch of pitch. In my experience with larger props (14x26) that the rule generally holds true, with some variation on cup and prop design.

 

[Baby John]

If you buy a stainless power tech prop and don't like it you can return it for a fee of about $50 and swap it for one with a different pitch so long as you don't scar or scratch it before you return it.

I'd definitely drop down 1" in pitch but I don't think I'd go lower than that. I know there's a formula for the calculation but..sometimes that "Slip" factor that's also a part of the formula is just different and with just a 1" drop in pitch you'll change that "slip" factor as well.

Besides everyone who owns a boat wants it to go as fast as it can go regardless of whether or not it's already going "fast enough". I do at least.

Call Power Tech and tell them what you're doing. They have a dozen different styles of props and they'll listen to your description and will make the best recommendation for you. They know what they're talking about.