Need Help getting a new prop please.. I am definetly confused.

[Mybadomen]

Hi guys and thanks for any help in advance. Before i go out and spend money on the wrong Prop i really need some help on getting the right one.

Here is the Background of the boat -

I bought this boat about 3 weeks ago. Its a 16 foot Aluminum Grumman boat bow rider with full interior ,windshield ,on board gas tank , wooden floors and other stuff that adds weight so i think its heavier then i would think for a 16 foot aluminum boat. The person i bought the boat from said it came with a 40hpthat went bad so they bought a used 1987 55 hp Evinrude 2 cylinder 2 stroke engine. From what he told me he never had any power in the boat since the engine swap and is the reason he actually sold the boat to me. I went through everything Carbs , ignition ,everything i could think of and the boat would not go over 3000 rpm. with me alone in it i think i got to max 3200 rpm roughly. So i was thinking a cylinder was cutting out or main jets were clogged but nothing fixed it . The boat goes out and runs all day perfect accept for the fact i cannot get over 3000 rpm which means i cant even get on plane,This is very dangerous where i use the boat because its hard to get through under the bridges in my town when the current is running and i have no power . anyay before i write a whole book here trying to explain i will get to the point.

- 1987 Grumman16 foot Bow rider aluminum boat with full interior
- 1978 Evinrude 55hp 2 cylinder 2 stroke engine with 12 x 17 prop
- In Neutral the engine will hit 5500 rpm which is what it should max out at.
- In gear Max rpm is 3000 rpm.
- i usually have a full boat . 6 adults but even alone in boat is max 3200 rpm and cant plane.
- the prop on the boat now is a 12 x 17 and looks way newer then the engine so i would say its been replaced.

So here is where i am totally confused . and scared of buying the wrong prop.

What i read and im sure im wrong is that
- changing the diameter of the prop by 1 inch = 500rpm ?
- changing the pitch of the prop by 1 = 200 rpm ?
- i need 2500 more rpm tp reach max rpm

According to that math this is what i come up with which i dont think is right?

- 12 x 10 prop would be roughly 4200 rpm wot ?
- 11 x 10 prop would be 4700 rpm?
- 10 x 10 prop would be 5200 rpm ?
- if stayed at 12 inch diameter i would need a pitch of only 7 pitch which dont sound right ?

I am totally confused on what prop i need to achieve 5500 rpm which is 2500 rpm more that i need .

I really hope someone can shine some light on this for me because i went to a couple stores and they i got 3 different opinions on sizes .But it still dont add up . the math dont at least .

1) 1 recommended either a 13 x 11 ? thats an increase in diameter and drop in pitch but i dont see how they get 5500 rpm by doing that ?

2) another shop recommended 11 3/4 x 13 ? that sounds closer but still not 5500 rpm ?

3) and last shop recommended 12 x 13 ? to me that only on paper would bring the boat to 3800 RPM ?

I am so Lost . Please help .

Thanks Dave

 

[alldodge]

Have you checked compression?

 

[Mybadomen]

Have you checked compression?

Yes the compression was just checked again this week , my buddy checked it for me and said its around 115 - 120 which he said should be fine for a 1978 motor . I am sure its a little week. i am trying to buy a prop now to hopefully resolve the issue but im scared of just jumping in and buying the wrong prop . I found a 12 1/2 x 8 prop which i think would brin the rpm up to 4800 rpm roughly but again im not sure .

 

[dingbat]

Prop selection tools give a range of props from 12.75" x 17" to 14" x 11" for that motor.

Pretty large discrepancy between recommendations what you've experienced.
Are you trimming up and down correctly?
Has the lower unit been swapped out?

 

[racerone]

There is no way that you need a 8 " or 7" pitch prop.-----STOP your guessing and have the motor checked out.-----I say it is running on just 1 cylinder or other problems.----Compression should be about 145 PSI or so.-------Pull the cylinder head and stop hoping that there is an easy fix here.