Tractor Forum banner

1 - 14 of 14 Posts

·
Registered
Joined
·
1,310 Posts
Discussion Starter #1
Would it be possible to put a conventional engine such as a briggs in place of an out board motor and just use the lower unit?
I ask this because the lake we hunt is swan lake and its low water levels so the water pipe gets clogged all the time. I know briggs makes a 5 horse model thats aircooled but thats too small. I was thinking if the crankshaft is the same size if this project would be possible.
Ryan
 

·
EX Super Mod
Joined
·
5,317 Posts
Get you one of these they use them alot here in the swamps.

go devils

<img src=http://www.go-devil.com/Images/Gen6a.jpg>
 

·
Registered
Joined
·
8,567 Posts
Originally posted by farmallmaniac
Would it be possible to put a conventional engine such as a briggs in place of an out board motor and just use the lower unit?
I ask this because the lake we hunt is swan lake and its low water levels so the water pipe gets clogged all the time. I know briggs makes a 5 horse model thats aircooled but thats too small. I was thinking if the crankshaft is the same size if this project would be possible.
Ryan
Ask and Ye shall receive. :D ;) :winky:

Briggs & Stratton 5 HP 4 Stroke Outboard Motors
 

·
Registered
Joined
·
289 Posts
I guess it would be possible to do that. Outboard driveshafts are splined and fit in the end of the crankshaft. The engine would need to be lined up just about perfect to avoid snapping driveshafts. Depending on what mid-section and lower unit you use, something may have to be rigged for shift linkage. I wonder about rpm though. Most outboards are designed for wide open throttle rpm between 5000 and 5500, so the gearcase and props are designed for the same. The briggs would have a harder time moving the boat than the original powerhead because of the lower rpm. 15hp @ 3750rpm (or whatever the briggs is rated at) is not the same as 15hp @ 5200rpm.
 

·
Registered
Joined
·
1,077 Posts
Here in sunny Fla. wed use a jack plate to raise the motor up between 4-6 inches and seems to work well in bayous and low water arond this area.
 

·
Premium Member
Joined
·
1,394 Posts
Sure it is, anything is possible with a little imagination. I worked with an older fellow who retired years back that lived on a major stream just off the Alabama River. He was a lot like me in terms of conjuring up odds and ends. He had a number of old outboard lower units he collected over the years that were trashed when someone blew an engine etc, and added various Briggs & Strattons on them for the power source and they all worked just fine. Some may not have looked all that pretty, but the main thing was thy were functional and worked great. He used them all the time setting and running trot lines, and jug fishing etc.........
 

·
Premium Member
Joined
·
1,394 Posts
Originally posted by aguysmiley
I guess it would be possible to do that. Outboard driveshafts are splined and fit in the end of the crankshaft. The engine would need to be lined up just about perfect to avoid snapping driveshafts. Depending on what mid-section and lower unit you use, something may have to be rigged for shift linkage. I wonder about rpm though. Most outboards are designed for wide open throttle rpm between 5000 and 5500, so the gearcase and props are designed for the same. The briggs would have a harder time moving the boat than the original powerhead because of the lower rpm. 15hp @ 3750rpm (or whatever the briggs is rated at) is not the same as 15hp @ 5200rpm.
I would think horsepower is horsepower. (proovided that both manufacturers rated them the same) The big difference is the rpm range or power band in which the motor gets the rated horsepower. I would bet that the B & S engine would swing a higher pitch prop without getting lugged down than a comparable 2 stroke would, so you should be able to pitch the prop up on the B & S to improve speed, and run it at lower rpms due to it having more torque. Most 2 strokes need to be run wide open as thats about where their max power band is at. The real nice thing is to be able to slow it down to slow speeds without bogging down and choking up and such like a lot of 2 stroke motors do.

The same indiviudal that I know that has a few outboards adated to B & S engines also has a troling motor made from a weedeater......he just substituted a trolling motor prop for the cutter head on a Poulan brand weedeater, and it works great. Much noiser than the typical bat powered troling motors, but then this thing probably did not cost him one cent to make as he is even more frugal than I am. As frugal as I am I still like for whatever I cobble together to have at least a sense of not looking all that much homebrew if possible.
 

·
Registered
Joined
·
8,567 Posts
Originally posted by farmallmaniac
chief do they have any larger models?
Ryan
Right now I think Briggs just has the 5 hp model. They are a heck of a lot cheaper than these others. For that matter, you could mount 2 of the Briggs outboards for a 1/4 of the price of one of those others.
 

·
Registered
Joined
·
289 Posts
Originally posted by Chipmaker
I would think horsepower is horsepower. (proovided that both manufacturers rated them the same) The big difference is the rpm range or power band in which the motor gets the rated horsepower. I would bet that the B & S engine would swing a higher pitch prop without getting lugged down than a comparable 2 stroke would, so you should be able to pitch the prop up on the B & S to improve speed, and run it at lower rpms due to it having more torque. Most 2 strokes need to be run wide open as thats about where their max power band is at. The real nice thing is to be able to slow it down to slow speeds without bogging down and choking up and such like a lot of 2 stroke motors do.

The same indiviudal that I know that has a few outboards adated to B & S engines also has a troling motor made from a weedeater......he just substituted a trolling motor prop for the cutter head on a Poulan brand weedeater, and it works great. Much noiser than the typical bat powered troling motors, but then this thing probably did not cost him one cent to make as he is even more frugal than I am. As frugal as I am I still like for whatever I cobble together to have at least a sense of not looking all that much homebrew if possible.
You're right on a lot of your thinking here. The B&S would need more prop than than the original 2-stroke would to achieve similar top end speeds. The problem is that it would be harder to get the boat on plane with a larger prop.

I'm not the greatest at explaning things. In fact I kinda suck at it, but here goes. Horsepower is just a number derived from torque.
It's actually torque multiplied by r.p.m divided by 5252. With this you can figure that an engine with 15 horsepower at 3750 actually has more torque at that r.p.m. than an engine with 15 at 5200 has at 5200. Logic would tell you that 15 at 3750 would be better. The thing about horsepower though is that it's based on r.p.m. Most outboards run a gear reduction somewhere between 1.6 to 1 and 2 to 1. When you put torque through a gear reduction, it multiplies the amount of torque by the gear ratio. Example: 20ft.lbs. with a 2 to 1 ratio equals 40ft.lbs. Once again 15 at 3750 looks better. The difference is that the slower winding engine would need a higher gear (lower numericly) to achieve the same top speed as the higher winding engine, or it would need more pich on the prop. Putting a bigger prop on the engine is effectively changing that gear ratio, just like putting taller tires on a car. This means that the higher you make your power, the more torque you will have where it counts because you can use a lower gear (higher numerically) or a smaller prop. The lower the final gear ratio, the faster you will come out of the hole and get on plane.

I'm a mechanic in a marina, and I see a lot of 2 and 4-stroke outboard engines. I can tell you one thing. Nobody makes a 4-stroke outboard that will compete with a comparible hp 2-stroke.
It's not even close.

I'm not saying that setting a briggs up on your outboard won't work. It's just not going to have the same performance as the 2-stroke. I understand the reasoning for wanting to do it, and it makes sense. If you're just looking to get through some muddy water and hunt ducks, it should work. If you're looking for serious speed, I don't think it would do well. I would actually keep the prop that was originally on the engine to take advantage of the gear ratio, and just not worry much about top speed.
 
1 - 14 of 14 Posts
Top