I am looking to see if any one would know or know where I can find what kind of speed with a out board. I am wanting to cruise at 10 MPH. How much HP would I need?

IN MY OPINION, 10 mph is not a good speed for a 35' boat. It's faster than displacement speed (about 7 mph) and too slow to plane. If you're willing to settle for displacement speed a 100 hp outboard should do it easily. Getting that extra 3 mph is going to take a LOT more power (and gas).

My 35' boat will cruise all day at 7 mph, on a 120 hp diesel (inboard) running about 1/2 throttle. Even wide open I can't get 10 mph.

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Willie
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw

Thanks stmbtwle. That is what I wanted to know. I towed the house boat for 70 miles with my bay boat (275 hp) and at 2300 rpm we cruised at 7 to 7.5 mph. when I brought the rpms up to 3500 I can only tow the boat at 8.5 mph. I did not know if it was because I was towing the boat but it seemed that the boat wanted to cruise at 7-7.5 mph. I was just hoping that I could go a little faster. I think I will look at getting a 140 to 150hp 4 stroke engine to put on it.

Also I found a 3.0 Mercruiser that is like brand new. Do you think it would push my boat at lease 7 mph?

Your experience towing the boat is a good start...

I don't know what a 3.0 equates to in respect to horsepower, but my guess it's big enough. A bigger engine loafing is generally better than a small engine pulling it's guts out. It's easy to see when you've hit the "wall", the wake will become noticeably larger with RPM but very little increase in speed. Hull speed is what it is; pushing the issue only wastes gas ($$$).

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Willie
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw

Edited: 08/23/2011 at 09:37:25 PM by stmbtwle

Thats. The 3.0 engine has 120 hp.

If the price is right it might be a good choice. I expect the Mercruiser swings a bigger prop than a comparable outboard and that would be a good thing. 7mph is probably close for cruise and it should be great for goofing around. Bear in mind that most houseboats aren't about speed, they're about comfort, and 1-2 mph doesn't really matter. There are a few who will disagree but they have more gas money than I do.


If you feel the "need for speed" you still have your bay boat.

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Willie
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw

What outdrive are you going use with the 3.0? You can use an Alpha I or II or a Bravo II. You need to make sure you have the appropriate reduction in the outdrive.

The 3.0 will push you up to your hull speed - which seems to be about 7 mph.

The engine has a Alpha I outdrive. I am going to purchase the engine to try. Its for a coworker and has under 100 hours. It was in a boat that got hull damage of the trailer and he took the engine and drive out and built a stand in his shop for it. I went last night and he ran the engine with some outboard engine muffs and everything was good.

How can I tell if I have the appropriate reduction>?

What kind of a boat did it come off of? If a small runabout it may NOT have the reduction you want. I don't know; but there are those on here who can tell you what you have to do.

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Willie
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw

I never saw the boat but was told it was from a 19' ski boat. Here is 2 pictures that i took:

If the outdrive came with the 3.0 odds are high the ratio is 2-1. I have some experience with everything mentioned here, so here goes

A 3.0 pushed our 2007 Bayliner 175 around great, hit 44 mph once at just under 5000
with the stock 15x19 prop.
The X factor: the Bayliner only weighed like 1800 pounds...

Our SkipperLiner is essentially flat bottomed, drafting 14" or less.
It's pushed around by two 2.5 Mercruisers, just smaller versions of the 3.0
The props we got with it were 16 x 9 pitch (16" just barely fits the Alpha),
first year out I hit 9.9 mph on the GPS at WOT, about 3800 rpm (4400 altitude),
cruise speed was 6.8 to 7.2 GPS at 2800-3000 rpm. Great gas mileage.
I now have 15x13" pitch props that were tweaked back to 12 (so I was told),
the boat now hits 7 mph at 2400 rpm (see the pic), but my max rpm is only about
3400 with speed hitting about 9 mph. Props are a bit steep, but that's a whole nuther story...

Our Party Cruiser is a 32' twin-log pontoon with a cabin & full canvas, and it's pretty
heavy, could use a 3rd log. The logs average about 55% submerged at rest, so it
plows pretty good. It's running a 115 hp Mariner 2-stroke outboard,
and it cruises around 10 mph at 3800-4000 rpm with a 13x13 prop.
WOT I almost hit 14 mph once, but with some weight gone it would probably hit 14+...

SO-- if your boat is 35', the 3.0 should get it pretty close to 10 mph, just be sure to get a "pusher" prop for it.
And pretty much anything over 100 hp in an outboard should be pretty much the same story.
X-factor: the TWO Mercruisers in our 36,000# houseboat burns about 70% of the gas that the
ONE outboard on our Party Cruiser burns. If you can use an I/O, I'd avoid the outboard!
(but that's me)

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1988 SkipperLiner 53x14
1995 Tracker Party Cruiser 32 *for sale*
2003 Chaparral 260 SSI
2000 Allegro Bus 40' DP

Edited: 08/24/2011 at 02:51:17 PM by FranticallyRelaxing

I believe that there were four different gear rations for the Alpha. You would want the highest numerical reduction - 1.94 would be better than 1.81 - to help give the 3.0 more grunt. I will probably be one of the two I listed above. Either will probably work. Just get the biggest diameter prop with the right pitch that you can fit.

Thanks for all the information. I should have the new transom and stringers fiberglassed in this weekend.

Any tricks or special tools I need to know about to install the engine?

Just a update. I have the hull almost ready to be painted and plan to do so next weekend and have my Mercruiser put in. I called around yesterday and spoke to a couple of shop and they were all saying a 16x10 prop. I called Propco and they said the same.

Just wanted to get some advice from you guys if I should try the prop that came with the motor are just go ahead and buy a 16x10 with the reverse cup?

If you HAVE the prop that came with the motor then try it (new props aren't cheap). Even if it doesn't perform the way you like it'll give you a baseline to work from, and you might be able to get it reworked for less than the price of a new prop.

If you don't have any prop at all then go with whatever is recommended.

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Willie
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw

16 in diameter is probably the biggest that can fit. 10 in pitch is about as low as they make. I would try the prop that you have. But, if it came off of a runabout, it is probably 17, 19, or 21 pitch and will probably work like crap. So, don't make any judgement on ultimate performace based on that prop.

I had 16 x 9's on my Alpha's, left about 1/16" clearance. I couldn't find replacement 9's on the planet, and Propco is the ONLY place I found 10's. Everyone else, 11's is as low as they go...

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1988 SkipperLiner 53x14
1995 Tracker Party Cruiser 32 *for sale*
2003 Chaparral 260 SSI
2000 Allegro Bus 40' DP

Aluminum props can be repitched (within limits), and 1" is workable, maybe even 2". An alternative is to reduce the diameter some, a good prop shop can do that for you, too.

With only 1/16" clearance I'd think you'd have vibration issues.

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Willie
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw

I do not know much about houseboat props but can some one explain the difference of a large prop with not a lot of pitch vs a small prop with a lot of pitch.