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View Full Version : Suggested Prop Size For My Fantasy Houseboat 5.0L Mercruiser, 100 feet long 19 wide



Univoftnvol
02-06-2017, 10:52 AM
Hearing lot's of different suggestions from prop shops, rather hear from the users.

42gibson
02-06-2017, 05:10 PM
i would contact one of the manufacturers.

Frantically Relaxing
02-06-2017, 07:42 PM
Our SkipperLiner, 53' and weighs in at 36k#, came with 120hp Merc 4-bangers spinning Alpha One drives and Michigan un-cupped props, 16" dia x 9" pitch. Those props got me to 10 mph at just under 4000 rpm. Props weren't all that clean, could've maybe gained 100 rpm or so with some fresh paint, but there was no point. Cruise speed was 7 to 7.5 at 3100-3300 rpm.

So then one day I forgot about the props sitting in a bucket of muriatic acid & water... oops ;)

Trying to find 9" props is tough, I know there's that one place that will make you pert near anything, but we were at the boat show and we came across some prop guys. Nope, nothing in a 16x9, but we can get you a pair of cupped 15-1/2 x 11's. Fine by me, they'll push the boat around..

Then I get a phone call- props came in! But they're 13's, not 11's, and no 11's for a month or two. We can tweak them back a tad into 12 territory. And we'll knock a few bucks off.

Send them along, can't go boatin' without 'em!

So here's the thing. These props by all 'normal standards' shouldn't be on this boat. But guess what-- The boat maxes out at 10mph. But now it's doing it at just under 3500 rpm. And my 7 to 7.5 cruise speed happens at around 2700 rpm. And as a bonus, the 2700 rpm seems to be a great torque range for these motors, so I'm not 'lugging' the engines, and I'm actually using less fuel. If I'm trying to beat a storm, running at WOT won't hurt anything, the little 4-bangers will give me what they got and no more, and at the low RPM's the carbs don't have enough air moving thru them to overload the engines with fuel.

Obviously this setup wouldn't work with a planing boat, but for a 10 mph brick, these wrong props work great. And as a final bonus, in the harbor these things BITE, this boat is vastly more responsive to throttle changes than it ever was with the 9"ers.

So I get the same speeds using less fuel, and waaayyy better harbor maneuverability using props NO one would advise using. Even I won't advise buying steeper props-- but I would suggest maybe renting some.. ;)

easttnboater
02-07-2017, 06:13 AM
What outdrives do you have? I am assuming Bravo II's. I run a set of counter rotating 3 blade 19.5 in diameter x 13 pitch props on my 18' x 80' houseboat. I can turn 4,500 rpm and hit around 13 mph. I would say you should have 20 in diameter x 11 pitch props on your larger boat. Mercury Quicksilver 8M0078084 right hand rotation. Mercury Quicksilver 8M0078085 left hand rotation. Unless the prop shop caters to houseboats, they do not have a clue as to what you need.

Endurance
02-07-2017, 08:20 AM
Finding props with low enough pitch for houseboats can be a challenge if you're buying from someone that deals more in planing runabouts. For a houseboat, I have had good luck with Propco, and in particular their reverse pitch props. Their claim of 200% reverse thrust without hurting forward thrust was true for my boat. Since they do a lot of business in houseboat props, I also trust their advice about prop size and pitch. Link: http://www.propcopropellers.com/mercruiser.html (you will want to scroll down to the houseboat/workboat section for the houseboat props)

OLD HOUSEBOATER
02-10-2017, 06:02 PM
Heres the deal. The religion is: Prop your boat so that the engine can reach its maximum recommended RPM at wide open throttle. For pleasure boats that are used for high speeds this is GOOD! How ever for house boaters that never run wide open and work boats, another path exists. If you only cruise, over propping has some distinct advantages. More diameter and/or pitch will slow the engine down and result in a lower noise level, much better backing response, and in many cases greater economy. Many work boats and fishing trawlers are SERIOUSLY over propped for economy's sake. Since a houseboat doesn't ever get on plain and there is no need to "get over the step" Over propping is a viable route to consider.

What say you

Rodney

Endurance
02-13-2017, 12:33 PM
OHB is spot on, as usual. The advice he gave is exactly what I got at Propco. The folks at Propco added one caveat: Overpropping works well so long as no one pushes the motor(s) beyond a comfortable cruise RPM. On my boat, that is around 2,800 or 3,000 RPM. If you're overpropped and someone pushes the motor beyond whatever comfortable cruise is for that particular motor or motors, you have a situation where you're pushing to achieve an RPM that the motor can never reach because it has too much pitch. That lugs and can damage the motor(s).

I can think of two situations where you could lug and damage the motor. One is if you have a multi-owner or multi-user boat where one user just doesn't know better. The second is a seasoned boater pushed by something like a medical emergency or a sudden storm. I hope I'm never in the "don't know better" first category. I pray I'm never in the second category where minutes matter and I'm willing to burn some extra fuel to get somewhere a little faster. But you never know if you might find yourself in a spot where the weather forecasters were wrong, someone takes ill, or some other emergency arises. For that reason, I chose to overprop my boat a little but not so much that I would damage my motors if I ever had to go for a time at wide open throttle.

Thankfully, I have never had cause to have to go above a cruise RPM that takes me along around 6 MPH with a nice low fuel burn rate. But if I'm ever in a pinch and need 10 MPH, I am glad to have it available for only the cost of burning through some extra gas.

Univoftnvol
02-14-2017, 10:35 AM
Yep Bravo II

Univoftnvol
02-14-2017, 10:37 AM
Perfect thanks much

Bamby
02-14-2017, 12:36 PM
OHB is spot on, as usual. The advice he gave is exactly what I got at Propco. The folks at Propco added one caveat: Overpropping works well so long as no one pushes the motor(s) beyond a comfortable cruise RPM. On my boat, that is around 2,800 or 3,000 RPM. If you're overpropped and someone pushes the motor beyond whatever comfortable cruise is for that particular motor or motors, you have a situation where you're pushing to achieve an RPM that the motor can never reach because it has too much pitch. That lugs and can damage the motor(s).

I can think of two situations where you could lug and damage the motor. One is if you have a multi-owner or multi-user boat where one user just doesn't know better. The second is a seasoned boater pushed by something like a medical emergency or a sudden storm. I hope I'm never in the "don't know better" first category. I pray I'm never in the second category where minutes matter and I'm willing to burn some extra fuel to get somewhere a little faster. But you never know if you might find yourself in a spot where the weather forecasters were wrong, someone takes ill, or some other emergency arises. For that reason, I chose to overprop my boat a little but not so much that I would damage my motors if I ever had to go for a time at wide open throttle.

Thankfully, I have never had cause to have to go above a cruise RPM that takes me along around 6 MPH with a nice low fuel burn rate. But if I'm ever in a pinch and need 10 MPH, I am glad to have it available for only the cost of burning through some extra gas.

I'm sorta in the same boat myself, but at least in my case you overlooked a third situation. In the waters we boat in boat bottoms tend to get pretty nasty dirty over the boating season. So much so that physical speed is lost to water resistance from the filth. As such I've found I've got to keep a careful eye on my tachometer to assure the rpm's are still rising as the throttle is applied. Mostly I watch for rpm stall and hold throttle full at the found stall speed and run her ahead for awhile letting some of the crud wash off before playing around with the throttle again.

Anyway, as far as my motor goes I was never able to detect and felt lugging in it's performance. But what it would do is make oil which can also destroy a motor in pretty short order. It seems some of the extra fuel being injected into it couldn't burn was finding it's way past the rings and into the oil. Now that I'm tuned to my boat I know what to be aware or watch for and I'd suggest to anyone attentive that over-propping is a good thing but only for an real attentive skipper.