Need help choosing a prop!

[mitchbuck]

Well I think that I have finally worked out a lot of the bugs out of my 1990 90hp Force that I installed on my boat last season. I was out all weekend and it ran great! One thing that I would like to do is replace the prop, which is old and dinged up. Also, the engine came off an old small bayliner, and I don't think its the ideal prop for my boat as My cruise at 4K RPM is under 20mph (compression checks out and carbs were rebuilt last year). I have a 1984 17ft Proline, which is a really heavy 17fter (closer to 18ft) and weighs just under 2000lbs. I was trying to check what the prop size/pitch but can't decipher the letters on it.....in one spot it says "15" and in another location is says "C297".....so I am assuming that it is a 13.5 in x 15 Pitch. Does anyone have any recommendations on a size and pitch and brand/model of propeller to get? I'm not looking to break the bank and I want aluminum as well since I run in rocky shallow waters frequently. I see iboats sells props here, so might be the easiest route. Thanks!

 

[pnwboat]

Re: Need help choosing a prop!

Have you taken the prop nut off. Sometimes there are numbers on the hub that get covered up by the prop nut/washer.

You mentioned that you cruise at 4K @ 20mph. What is your maximum RPMs at WOT. Note, this may not be a accurate reading of the RPMs for your given diameter/pitch prop if it's really badly dinged up.

 

[mitchbuck]

Re: Need help choosing a prop!

My boat maxes out just under 30mph (~28knots) at 5k rpm WOT.....but it has to be flat and only myself on board. I'll try checking for numbers on the hub later though.

 

[Jiggz]

Re: Need help choosing a prop!

4K WOT RPM is a little low of course depending on the prop size. Unfortunately, there are three versions of Force 1990-90HP based on their ignition system and thus have three different WOT RPM's. If you have the Prestolite system it is rated at 4500-5500 WOT RPM, Thunderbolt is 5000-5500 while the CDM is 4750-5250 RPM. I would recommend doing the "Link and Synch" check on one of the sticky post and verifying timing before going any further.

 

[mitchbuck]

Re: Need help choosing a prop!

4K WOT RPM is a little low of course depending on the prop size. Unfortunately, there are three versions of Force 1990-90HP based on their ignition system and thus have three different WOT RPM's. If you have the Prestolite system it is rated at 4500-5500 WOT RPM, Thunderbolt is 5000-5500 while the CDM is 4750-5250 RPM. I would recommend doing the "Link and Synch" check on one of the sticky post and verifying timing before going any further.

WOT is just under 5k and I cruise at 3800-4000 rpms where I see 17-20mph at 2.2-2.5 mpg. I already checked the timing and have done the link and synch 3 times in the past year so I am set. I have the ignition system with the blue coil boxes, which are the US Marine ones not mercury (can't remember if its thunderbolt/prestolite/CDM)

 

[pnwboat]

Re: Need help choosing a prop!

You have the Prestolite ignition system. Looks like those Prolines are deep v-hull big water fishing boats. I can see why they weigh almost 2000LBs. I suspect that a 13.5 X 15 pitch prop is going to the one you want.

 

[SkiDad]

Re: Need help choosing a prop!

15p sound right if it's heavy. normally i would suspect a 17p on a lighter boat but your WOT is about right.... if you have the water pickups on the bullet and need a new prop the michigan number is 072055 - i have 2 of those and they run nice. if your water pickups are above the bullet you need a different prop number but can find them here on this site.

 

[mitchbuck]

Re: Need help choosing a prop!

15p sound right if it's heavy. normally i would suspect a 17p on a lighter boat but your WOT is about right.... if you have the water pickups on the bullet and need a new prop the michigan number is 072055 - i have 2 of those and they run nice. if your water pickups are above the bullet you need a different prop number but can find them here on this site.

By bullet do you mean the portion of the lower unit housing housing the gears directly behind the prop? If so, that is where my water pickups are.

 

[SkiDad]

Re: Need help choosing a prop!

By bullet do you mean the portion of the lower unit housing housing the gears directly behind the prop? If so, that is where my water pickups are.

the props that are closed hub fit only on the ones with the pickups just behind the prop (on the rounded section) - if the water picks are above the rounded section in the flat area you need a thru hub prop. You can use a thru hub on either style lower unit but the closed hub can only be used on the older style lower unit.

here is my lower unit and prop - the pickup in on the bullet and can use the closed hub prop i mentioned above
View attachment 218720

one prop that I've thought about getting is a hustler prop - they are only about 100 bucks and they have a 13 or 15 that might work for you.

 

[mitchbuck]

Re: Need help choosing a prop!

Thank you for the pic, that is what I have!

 

[MickLovin]

Re: Need help choosing a prop!

May I ask what it is you use the boat for? Do you want to use it for towing skiers or do you use it to go deep sea fishing?
So do you require more hole shot speed or overall MPH improvement?
I think this should have been posted in the prop section of the forum personally.
There is a lot more to prop selection than I first realised after reading the prop section of the forum, I have changed from my 23" ally to a 21"s/s and now I have a 17" s/s. But now Frank has me unsure of my engine HP as it is giving out more with the 23" than it should comparing RPM to MPH. Just thought I would give my 10 cents worth hope it helps.

 

[mitchbuck]

Re: Need help choosing a prop!

I didn't even realize that there was a prop forum, thanks! I use the boat mostly for fishing. Don't care about hole shot that much though right now with 3 people on the boat it takes a bit to get up on plane....really would like to gain some mph at cruise.

 

[MickLovin]

Re: Need help choosing a prop!

If you go down in pitch you will only loose from my understanding of the changes in pitch, Frank mentioned that the new 17" prop I bought was for bow raising, maybe that would be something to look at for hole shotting better but still keeping a decent speed. You mention you have a 15" and obtain 5k RPM @ WOT you could possibly try a 17" like mine but there are others who would know more and have done a lot of research which is posted in the prop section and is very interesting reading.
To be honest, I think I lucked out with my 17", as it is the prop specified for my motor and I am sure my RPM will return to 5k+ as it is only doing 4.5k I think. Also I am grabbing a new portable Tacho to calibrate my existing one, so make sure your TACHO and SPEEDO are correct
Good luck with it.

 

[mitchbuck]

Re: Need help choosing a prop!

Thanks for the input. Here is my prop. Its a 15 pitch but the blades are oddly shaped and I'm wondering if that is part of the problem (in addition to wear/damage).

 

[MickLovin]

Re: Need help choosing a prop!

Ask the blokes in the prop thread they will know for sure, or someone here may chime in. I am definately no genius when coming to what prop to use, I only have gone on what I have and what should be on it and I know the 23" prop was wrong as the rpm were too low.

 

[MickLovin]

Re: Need help choosing a prop!

Ask the blokes in the prop thread they will know for sure, or someone here may chime in. I am definately no genius when coming to what prop to use, I only have gone on what I have and what should be on it and I know the 23" prop was wrong as the rpm were too low.



These are the 3 props I have now, the first is the new 17" the second is what came off my 75HP chrysler which was a 21" s/s and the last is a 23" ally originally off my 100HP if it is really a 100hp as the motor has had a reco and can't tell if it is the original block.

The 23" is solid hub the other 2 are thru hub

 

[jerryjerry05]

Re: Need help choosing a prop!

If your getting 5000 at WOT then you should probably stay with the prop you have.
Going to a 17 from a 15 will maybe give you a bit more speed? But your economy will suffer.
I changed from 17s to 19s and got about 3mph but the fuel usage went down 20%
The load in your boat will affect the performance.
2-3 people a dog or 2 and a full cooler or 2 and full tank will affect the performance.
Being as it's an older boat it's possible the foam floatation is saturated.
I cut up a 21 ft. I found about 800# of saturated foam.
The pic of the prop, it doesn't look that bad.
It could use a rebuild or hammer the edges smooth and a file to take off the rough edges.

 

[Big Fish Billy]

Re: Need help choosing a prop!

Another option is a four bladed prop. You'll still be able to wind it up tight, but who is going to run there all the time. You want to find a nice speed on plane, but economically. You'll get a few mph less, but you'll be able to push that "2-3 people a dog or 2 and a full cooler or 2 and full tank" with more power and mpg.

 

[MickLovin]

Re: Need help choosing a prop!

If your getting 5000 at WOT then you should probably stay with the prop you have.
Going to a 17 from a 15 will maybe give you a bit more speed? But your economy will suffer.
I changed from 17s to 19s and got about 3mph but the fuel usage went down 20%
The load in your boat will affect the performance.
2-3 people a dog or 2 and a full cooler or 2 and full tank will affect the performance.
Being as it's an older boat it's possible the foam floatation is saturated.
I cut up a 21 ft. I found about 800# of saturated foam.
The pic of the prop, it doesn't look that bad.
It could use a rebuild or hammer the edges smooth and a file to take off the rough edges.

Thanks for posting that comment about fuel consumption jerryjerry, I didn't even think about that, when I have now changed from the 23' Alloy to a 17" s/s, I am very interested to see what fuel consumption I will have.