Repower, need prop ideas

[Mechmagcn]

I am current working on a new to me daily driver. Boat is an aluminum flatbottom pilothouse jonboat, 18', 54" bottom and 20" sides. I figure it weighs less than 1250lbs. I'm repowering it with a Mercruiser 140hp 3.0 and Alpha One drive. The drive has a Solas 14.3X23 aluminum prop. I'm sure this is way too much prop for this engine/ boat combo. Any ideas for a baseline would be appreciated.

[WesNewell]

All I see is a 130hp with a 1.94-2.0 gear ratio depending on the year model. In any case, a 23P is the recommended prop pitch for 800# load with 20 gal. fuel for a 2.0 GR. Should be ok for all GR. assuming your boat weight is indeed no more than 1250# for a total weight of 2800# with engine, fuel, and 800# load.

[Bondo]

I'm sure this is way too much prop for this engine/ boat combo. Any ideas for a baseline would be appreciated.

Ayuh,...... Welcome Aboard,..... Try what you've got,...... then adjust, startin' with a known value,....

[Mechmagcn]

Hope to get it in the water in a few weeks, we'll see what it does then. I know the previous owner of the ski boat the power train came from said the prop was too much for it, but that was an '87 Avanti 170DL fiberglass boat. I'm not sure of the year model of the engine, he replaced it not long before finding the rotten transom. The stickers on the valve cover say 140, but he did say that cover was from the old engine and this one was a higher HP.
Thanks for the response, I'll update after the splash.

[Scott Danforth]

Its a 3.0, cant get much more HP that 140

[ahicks]

An I/O sounds like a lot of weight on the back of a boat like that. Hope is it's wide and sturdy! 23p sounds like a lot of prop here too. Betting you wind up with a 17 or 19p.

[Mechmagcn]

I think the boat can handle the I/O easily. I was told that at one time it had a 115 outboard on it. The transom has been rebuilt with 1.5" of plywood sandwiched between 1/4" aluminum in and out. The front engine mount is tied into the transom and two stringers using 2X3" tubing, so I don't think there will be a problem there.
If other projects would stop getting in the way I'll get it in the water soon for a trial run.

[QBhoy]

I think you might be almost off to a good start. I’m aware of heavier 18 ft bowriders like bayliner 175 and maxum etc etc with the 3.0, 2-1 ratio and running well with a 23” not lightening quick out the hole but useable and fairly impressive too ends of mid 40’s gps. I think a 23” is a good start. I don’t think you would be more than an inch or two out on pitch, if at all.

[jimmbo]

What is the Gear Ratio of your Drive? Might as well get that info now. You should be able to find it on the trim or decal that wraps around the upper gear housing. As mentioned in the earlier posts, the 3 liter is pretty much maxed out already, and more recent MerCruisers had a 135hp rating.

A trial run with the 23" will give you an idea of where you are regarding propping. What year is the engine? That will come in handy determining the WOT rpm range it needs to be run, as thru the years the spec would change.

[Mechmagcn]

I have no idea what ratio the drive is, nothing on the sticker. The PO had changed the engine and used the valve cover from the old one, so no idea on the year model of it.
I'm ready to get this thing in the water if the weather will cooperate, seems to rain every day that I'm off.

[harringtondav]

Mechmagcn I remember your earlier post regarding keeping this (under consideration at that time) Stern Drivel upgrade freeze safe during occasional cold snaps. Now it looks like you've committed. What is your plan?


....I'm still a bit envious of a Hub/Wife that can commute to work on the water.

[Mechmagcn]

Still not quite sure on the freeze protection. Our current boat has electric hookup for a small heater on a timer to keep my wife warm on our commute, I'll probably use either a block heater or magnetic oil pan heater.

[QBhoy]

Still not quite sure on the freeze protection. Our current boat has electric hookup for a small heater on a timer to keep my wife warm on our commute, I'll probably use either a block heater or magnetic oil pan heater.

Hi
although a help, I wouldn’t rely on this. A few years ago round here, about 50 or so engines were freeze damaged when the a breaker failed in the local marina. All the guys who had only used heaters and not drained their engines were caught out. Just a thought

[Mechmagcn]

a breaker failed in the local marina.

Now that would be kinda ironic since I'm the head of maintenance at the state park where the marina is located at.

[QBhoy]

Haha. Keep that juice flowing then, for God’s sake !!