Prop help

[Ashman]

Can some of you guys school me in props? I am looking for a new/used one for my boat to have as a spare and I get confused about what I need. Seems to be a lot of choices.

My boat is a 1984 Thompson with a Mercruiser outdrive. Here is where I get confused and maybe it doesn't matter, but will a prop for any Mercruiser work on mine? Does it go by what the "name" of the unit is?

What pitch prop should I get to slow my boat speed down? Which one to ski behind?

Pic of what I have but I don't know the "name" of it.

Thanks in advance for the education.

[1mainiac]

Prop School first what is your current prop it will be stamped on the hub usually like 15X15 or 15X17 first number is diameter second number is pitch. Diameter give the amount of thrust area and pitch tells how many inches of water it will grab per revolution. Second what is your WOT rpm? You did not say what motor you have so assume a V-8 you should have a WOT range of roughly 4200 to 4800 rpm at full throttle with correct load and trim. If you RPM range at WOT is in this area you are pretty close to being the correct prop size. Next just because you are close to correct size does not mean it is the best prop amount of cup, 3 blade versus 4 blade all make a difference. A 4 blade would be better for water skiing as you will see better hole shots but will often reduce top end performance. Reducing pitch will get better take off but you need to make sure the RPM's won't exceed manufactures recommendations or you will find motor parts where they don't belong. As a rule of thumb 1 inch of pitch will make roughly a 200 rpm change if the cup and rest of prop factors are the same. 2 years ago I messed up my prop and had to run my spare which is 2 inches smaller in pitch thinking since my WOT range was about 4250 adding 400 would keep me under the 4800 max except the cup was different and my rpm's went to 5300. Also the boat was very fussy on plane and could not find a sweet spot. So spare prop went in yard sale and I got a good used prop for a spare and a new prop for the boat. Once you get the boat and prop matched if it is too fast to troll get bags or lower the engine rpm, bags would be better. Make sure you do all your prop testing with the boat setup the way you will use it because a couple hundred pounds will make a difference.

[Line Dancin]

Jim provided some very good info.

but call these guys:

http://westmichiganpropeller.net/index.htm

Tell them what kind of boat, engine and locate the part number stamped into the prop and they can tell you exactly what to use to achieve your means. This is also the best place to shop for props.

[Nailer]

Yes what they said ^^. There is other ways to slow your boat down though. For trolling you can drag trolling bags, or even 5 gallon buckets.

[Ashman]

Thanks fellas, I'll look for those numbers and post them here.

I have a 140 horsepower straight 4 cylinder GM motor. (wish it was a V-8 sometimes, but she do sip fuel with the 4)

I'm lousy with acronyms so here is another question....what is WOT?

[1mainiac]

WOT is Wide Open Throttle your range should be 4400 to 4800 rpm your prop is a Thru Hub Exhaust & 15 Tooth Spline, 4-¾ Gearcase style. My best guess would be a 14.5X19 3 blade prop. Which would give you Theoretical top speed of around 42 mph and 3200 rpm cruise around 26 to 29 mph. and on paper idle at 5 mph. I had a 17 with a 140 and my prop was a 13.5X19 and speeds were about the same.

[Ashman]

WOT is Wide Open Throttle

Well that makes sense.

I went out to the shop and got all the info:

Motor: GM 4 cylinder 140 (140 h.p. ?)

Outdrive: Mercruiser 4

Prop: Michigan Wheel 031026 13.75x21

It seems to me when we were on Big Bay de Noc last fall on calm water with my son and I, all our gear, our top speed was 35 ish. A little faster would be nice, but then I give up on the low end right?

I need to slow down to around 1 m.p.h. for pulling bottom bouncers and it seems I couldn't get any slower than around 2.5-3 m.p.h.

Suggestions?

Thanks!

[Nailer]

If you want to get down to 1 mph than trolling bags are the only way to go.

Call and talk with this guy.

Dick Boyajian. (231) 723-0013.

Or Amish Buggy Bags.

[1mainiac]

Ken is right on the bags best way to slow down with a 21 pitch you should see a little more top end do you remember what your WOT rpms were?

[Ashman]

Thanks again.

I've never used one (obviously) and I worry a bit about snagging my lures/harness on them or tangling a fish in them.

Would a Troll plate be an alternative?

[Nailer]

Thanks again.

I've never used one (obviously) and I worry a bit about snagging my lures/harness on them or tangling a fish in them.

Would a Troll plate be an alternative?

A trolling plate should work. I think I have 1, or 2 that came off my boat. I'll sell you them for next to nothing.

[Ashman]

I'd be interested.....sending PM.

[cary mac]

I run a 90hp with a trolling plate. I bought it a few years ago but never installed it. Put it on last year. Wished i had put it on win i bought it. I can run at .5 mph easy. There great for a 2 stroke. Helps keep the plugs from fouling out. Also it helps to get the boat up on plane. Draw back is making sure it is in the right position for what your doing

[1mainiac]

Only thing I don't like about trolling plates as they reduce thrust which means you lose some steer-ability meaning on a windy day it is going to suck. Honestly look into bags once you get them set correctly you will be glad you did as you can keep the boat squared up and even reduce roll. Also the first time you have go in and reset the trolling plate because it is bound up you will wish you had the bags.

[Line Dancin]

I'll agree with jim on this one. I personally hated the trolling plate on windy or choppy days. My dad thought it was the best thing since sliced bread. I do agree with bags helping to reduce roll on the boat also. They also help with fighting the steering wheel on rougher days. Try it though it may work just fine for you. I know a couple guys that love em and dont like messing with bags.