Prop sizing for a big heavy houseboat

Mercruiser service manual says for the 165hp engine, with a RPM spec of 3900-4300 I'd want a 16", 11 pitch, 3 bladed prop.
Under the "approx boat length" I fall under "houseboat", and it is a houseboat. Next column is speed range, listing 10-18mph
Row above is 30'+ 16-24mph, (I'm probably 36ft at waterline)
Row below is "work boat", 1-11mph.


Now, whether it's because it wasn't propped right originally, or because the engine is tired, I can't turn more than 3,600 RPM's, and doing that in ideal conditions I can just hit 10mph.

A few more specs on my boat-
LOA:38'
Beam: 11'
Approx weight: 15,000lbs

The workboat row lists a 16", 9 Pitch, 3 blade prop for the listed 1-11mph.


My understanding of all this tells me I should probably go with a lower pitch prop so the engine isn't straining so hard, but at the same time, I'd really rather not loose any more speed.

The lower pitch should let me gain back some revs, which should counteract the fact that I'm taking less of a bite I'm assuming. Should I expect to see an increase in range/speed with the lower pitch?

What about moving to a 4 blade prop?


Thanks for the help

Re: Prop sizing for a big heavy houseboat

That's a lotta boat for 165hp, soggy! There are way more qualified folks on this topic but I was way overproped and dropped from a 19 pitch 3 blade aluminum to a 14 pitch 4 blade stainless and felt like I got a new boat. Don't use my example for your situation but I'm sure you can improve performance without killing your engine. Your situation seems a natural for a 4 blade to me.

Re: Prop sizing for a big heavy houseboat

A 9p sounds about right. If you want to go to a four blade from a three, the consensus is to drop the pitch by 1. Your top end likely won't change noticeable but your "holeshot" will be better as well as your low speed control.

Re: Prop sizing for a big heavy houseboat

Heh.. Holeshot in the context of houseboats is amusing.


So, you guys think a 16" 4 blade with a 10 pitch would do it, or should I try for a 9 pitch like the "workboat" row in the shop manual?

Re: Prop sizing for a big heavy houseboat

Since the smallest pitch available is 11 for the Mercruiser, where do you intend to find these other pitches?

What size are actually running now? 11?

I see Solas offers a 15.5 diameter prop versus Mercury's 16 diameter. The smaller diameter will help gain some rpm(100-200 rpm).

A good prop shop should be able to re-pitch the 15.5 X 11 to a 10 pitch and remove the cup in the blades in order to end up with the equivalent of something like a 9 pitch.

Another factor to bear in mind is the condition of the hull in terms of the amount of marine growth on there. It doesn't take long to collect a fair amount of crud, which will affect the speed.

Re: Prop sizing for a big heavy houseboat

I get a thin brown slime, but otherwise, there isn't any substantial growth.

Hull condition as it is though, is not optimal. This 38ft boat was probably designed as a throwaway back in 1970-73 (somewhere around there). The superstructure was only plywood stapled as the seams. For the entire boat, they probably only used three 2x4's.
Get down to the hull, and it's sheetmetal over 1x box tubing. Not nearly enough to keep that hull in shape over years of sitting on blocks in the winter, especially when most of the weight ended up on the keel. Essentially flattened out the hull, and the skin has some bulges and dents.
But not much I can do about that, so we work with what we can and re-prop.

Wasn't limiting myself to Merc props, so whatever pitch I can find where ever that may come from will work for me.

I don't believe there are any prop shops around within a couple hundred miles, and I'd be more comfortable being able to walk into a place and talk with someone with the prop in hand, so I'll probably stick with buying a new prop.

Re: Prop sizing for a big heavy houseboat

If you're not going to be traveling to areas that have propeller shops, I'd just suggest you Google "marine propellers" and call one of the big, reputable mail order prop shops on the telephone.

They're more likely to have an ultra low pitch Mercruiser propeller. They can also do modifications to a prop that might make your boat perform better for your specialized use.

You're not even talking about spending very much money, as aluminum Mercruiser props are downright cheap.

Re: Prop sizing for a big heavy houseboat

Thanks, I'll make some calls and give it a shot.