Fast, Small, Boat Designed Around Mercuiser 140

Hello all,
I am brand new to the forum! I have a Mercruiser 140, 4cyl engine, the mating outdrive, and all the controlls from my old boat. Everything was in perfect running condition when I pulled it from the boat. I have decided that the old boat is too far gone to restore and would not be worth the money to put into an almost 30 year old boat. It is a 1984 SeaRay Seville that has everything mostly rotted out. I got to thinking (which can be dangerous) what can I do with the engine/equipment. I have rulled out dune buggy as I cannot readily attach a transmission to the engine. So that leaves me with building a fresh custom boat around this platform. I was thinking that if I build something around a single seat and the engine that I should be able to make something somewhat fast and manuverable. I like the look of the tunnel hull but from what I have read is that it can have aerodynamic issues. I am looking for advice and input on designs to get me started in the right direction. Techincal details are much appreciated as I am an engineer...
Thanks in advance!
John

 

John; Start with a statement of requirements (SOR). What do you want the boat to do? What is the main purpose of the boat? Where will you use it? In Cincy, I suspect that you'd use the river. ...............

Fill in some of the blanks and you will probably get suggestions from several knowledgeable respondents. You do have a lot of options, from the most basic of fishing boats all the way to three point hydros and beyond.

 

Messabout, thanks for the reply! I guess SOR is a good place to start. Yes it will be used on the river and other lakes in the area (Brookville). I think the main pourpose of the boat will be like a go-kart on the water. Not a fishing boat or skiing/tubing boat; something fast but also able to take a tight turn and handle midly rough water. I like the idea of a single seat cockpit type. I would like it to be relatively small, under 1500lbs would be best because then I could towe it with my jeep. Construction would have to be wood/fiberglass. I am very open to sugestions...

 

Take a look at this Glen L model. You can do it in wood with glass over and your engine would be great in it. http://www.boatdesigns.com/17-or-18-Slither-vee-bottom-whitewater-sled/products/872/

 

Hmmm. SIngle cockpit. Are you thinking of something like the old Chris Craft, Century, Riva sorts of classic runabouts? Single cockpit kinda' sorta' implies a lot of decking.. WIll you build the boat yourself? As an engineer, I suspect that you have plenty of knowledge about tools, methods and materials. You might be thinking of a "Cracker box" kind of boat. Scour the internet to see what's up with that venerable old design.

How fast is fast in your vocabulary? Thirty MPH +/- is achievable with a conventional runabout layout, Fifty or more may require some serious effort and design selection. It'll also involve some frustration and expense when fiddling with prop experiments and selection for a real go fast boat. The lower unit/gear housing, that you have, may be a limiting factor for the power that the engine is capable of developing.

A 15 or 16 footer is probably as small as you want to go. Either of those can be built within the weight target. Of course for "fast" boats, lighter is better. How much does the IO package weigh? What is the claimed HP output of the engine? What is the gear ratio of the lower unit.....etc. All that matters of course.

Lots of variables here. Please continue your descriptions.

 

A mini runabout is simple. Make a rough drawing of the engine/drive assemble to get an idea of the room you'll need, put a bow on it and start cutting wood. I'd add at least as much length as you and engine need to live within the boat. For example, if you, the engine and a fuel tank, need 8', make the boat 16'. This seems large, but remember you're dealing with a lot of weight in the engine/drive package, so it needs to be supported. Put a wing on it, paint Mrs. Budweiser on her side and have fun.

 

the 16' Donzi went great with a 140 MCM

 

That's pretty much the shape I'd look to do. Think of a high power rifle round, cut it down the centerline and maybe a V bottom, with seats on the opposite side.

 

A 4 cylinder 140hp Mercruiser wont be fast even if you mounted it on a jet ski !!! .
The power unit is way to heavy so you need a wide stern and some generous free board with a 15 degree dead rise about 14 feet long and just two seats . if you get to or exceed 50 mph you will be lucky !!

 

I would look at the 17 ft Boston Whaler as a sport boat with practical uses.

And some resale value.

 

I never heard of one with that engine installed. The Donzi 16 does not plane that easily...I would expect the 140 I/O to be pretty lame compared to the standard V-8 power that most came with.

I agree with what a couple others have posted...the old iron Chevy 140 is pretty heavy for what it puts out. Reliable?..very. High-performance?..not.

 

I disagree in that a 181 CID (3.0) GM I-4 is week kneed for it's weight (less than 400 lbs.). It's available with 180 HP, though more commonly with 140 or 160 HP and over 200 ft. lbs. of torque. With some minor hot rodding, it could produce a reliable 200 HP and the torque of a small V8. 160 to 200 HP on the butt of a modest V, 16' boat will scoot quite well. A deep V isn't a good idea, as you need more power to overcome displacement and angle of incidence loses, typical with these hull forms. I'd recommend an 8 to 12 degree monohedren, with well placed strakes or a fine entry warped bottom as better hull choices compared to a deep V.

 

I had three boats with 140 Mercs and worked on a few others.
A single 140 will work well in 15 to 19 feet fishing boats. I had a 27 with twin 140's.
Those are great engines with plenty of power throughout the RPM range.
I still have manifolds and Carbs left from some of those endeavors. I liked that engine.
Keep the Oil fresh and you'll never have any problems with the engine.

 

Yep, I've had a lot of luck with these in their various configurations too.

My estimates of a modest deadrise, 16' hull, with a 2,000 load and 10% slip, places it in the high 30's with a modest constant and low 40's with some tuning and a good prop. This means a hull stout enough to take a thrashing, with good performance too. Personally, I'd like to see a long and lean hull, say 20' with a 5' to 5' 6" beam, made light with close bottom longitudinals. She'd be faster and you'd have some elbow room too. Mid 40's on this type of power, is very respectable and would be relatively economical, if you cruised at 3/4's throttle..

 

^^^ Hear, Hear, for longer skinnier hulls. Economy of operation and ride quality is too often neglected until after the fact.