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New project. Ideals anyone

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wmlaks

Regular Contributor
I have been thinking over a new project for this year and was wondering if anyone has tried this before. I am thinking about putting a rotary engine out of a mazda RX-7 in a boat. The weight of these engines is about 200 pounds which I think would be very light for a boat engine and with a few modifications you can get a diecent amout of horse power from them. Base hp for a non turbo engine is between 100 and 135 depends on which engine variant you get. If anyone has tried this before or has any ideals please let me know thanks Michael
 
How you gonna couple it to the drive. How are you gonna cool it ?

What mani's and risers you gonna use.

When you get around just those things let us know.
 
Let's approach this from another angle!
Most good automotive type engines that are, or have been, suitable for a Marine adaptation, have at one time or another, found their way into the boat industry.
I.E., SBC, SBF, BB of both, Volvo's OHC 4 (6 in earlier days), GM's inline 4 (6 in earlier days), Chrysler's V-8's, and so on!

Question: How many of these Mazda RX-7 rotary engines have become OEM in any boat manufacturing? Even experimental?

If we were to give any credence to the boat industry's engineering influence, I think that we'd find that this engine is not suitable.

Note that there is very little "Off Throttle" or "Coast" time for a marine engine.... if any. These are under constant, and more heavy, loads once we leave the dock.
The gasoline piston engine must combate Detonation. I'm unable to comment on any Detonation issues with the rotary engine.
But all of this needs to be considered.

And Yes.... ditto the comment on Torque!
 
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Oh boy, I love a challenge.

But, the biggest thing is going to be how many revs, (or prop speed) can these OMC Legs take before they self destruct from overheating?

Most any powerplant can be adapted to the Bellhousing of the OMC Unit, but seeing as the Rotary relies on spinning a lot higher than say 5,000 RPM, you will need a smaller prop to allow the Rotary to get up to its' optimal 10 to 12,000 RPM.

The engine cooling will be the same for any other engine, but you will also have to incorporate an Oil Cooler.

Wouldn't be that difficult to design a Manifold Log, but making it would take a fair amount of time and using Stainless Steel would be the only way to go. You could even run the Exhaust out through the Transom, using a muffler and simply run the outlet down into the water to muffle it even further, but this will create a heat problem in the engine room.

Bruce.

PS. Used to own an RX3, and couldn't kill it, even with blown seals.
 
the_tassie_devil said:
  1. Oh boy, I love a challenge.
  2. But, the biggest thing is going to be how many revs, (or prop speed) can these OMC Legs take before they self destruct from overheating?
  3. Most any powerplant can be adapted to the Bellhousing of the OMC Unit,
  4. but seeing as the Rotary relies on spinning a lot higher than say 5,000 RPM, you will need a smaller prop to allow the Rotary to get up to its' optimal 10 to 12,000 RPM.
  5. PS. Used to own an RX3, and couldn't kill it, even with blown seals.
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  1. And a challenge it will be!:)
  2. A valid concern!
  3. With the aid of Borg Warner, and in AQ series Volvo Penta, and w/ many I/B's, yes..... but I doubt you'll find this for the Rotary engine nor the OMC Cobra.
  4. Disagree. The over-all drive ratio will require a drastic change. Lower ratios (higher numerically) offer a much smaller "drive" gear (relative to the "driven" gear), thus resulting in less gear tooth contact, effectively reducing gear set strength. Fact, not speculation.
  5. Try that with Marine loads! :)
I'm all for experimenting, and thinking outside of the box. But add the cost to experiment, and consider the likihood of failure............ I just don't think this is one of those scenarios that warrants experimenting.
JM2C's.
 
better idea - dealer near me has a NOS 2005 4.3 MPI for a nice price....droool:
digi34169943.jpg
 
If you follow the link you'll see that all the engineering has been done. The drop in rotary marine engine costs about 10 grand. For that stupid money you can have a big block, with a blower, and put out about 600 ponies all day long.
 
stupid is right
that's more than the 4.3MPI complete from water pump to propeller... transom assembly and all... :)
 
G'day Rick,

With regards to item 3 on your list, I didn't make myself clear in that anything can be adapted to anything, and a custom adaptor would be required, if one wasn't available commercially.

Where I live, one has to make up adaptors for most things that are out of the ordinary, or normal for that matter if living in USA where one can buy virtually anything, and not have to wait forever for it to arrive.

Bruce.
 
First I would like to thank everyone for their input. The boat I have in mind for this experiment has a 2.3 liter ford engine in it. The drive ratio for that engine is a 2 to 1 which I think will handle the higher rpm's of the rotary engine. I am planning to use a 13B engine which produces about 135hp and 133 pounds of torque. Through research I have found out that those numbers are the upper limits of the drive I plan on useing. The rotary engine weighs 170 pounds less than the 2.3 liter so I am hopoing that I will be able to reach faster speedsat simular rpm's the 2.3 wot is 5600 rpm's and the 13B makes the above stated hp and torque around 7000 rpm's. The oil cooler ideal is greatly apprieated I had not thought about that. I know that I will have to make a lot of the adapters to make this work. I am blessed with having a machine shop at my disposal. I am having trouble with how I want to do the exhaust. I have thought of a through hull exhaust but I would like to make the outsidde of the boat appear to have had no modifications to it. I want to use the rotary engine because they are a very tough little engine have a high power to weight ratio and they are starting to become popular in other hobbies. This engine is starting to be used in aircraft which also run the engine at 3/4 to full speed most of the time. Thanks again to everyone for their input
 
I think you need to understand how boats work. It's not always true that lighter is better. A boat makes a hole in the water, a hole that if filled with water would weigh what the boat does. That's the boat's displacement. There are two ways to move a boat thru the water...in it or on top of it. The two types of hull that travel this way are called displacement and planing, respectively. A displacement hull simply needs to push this hole around, so it only needs enough power to do that (it has a limit to it's speed based on waterline length, so adding power beyond a certain point is...blah blah blah). A planing hull needs far more power because it has to push the hull up out of the hole and then keep the boat moving fast enough that it does not fall back into the hole. The design of the hull is that of a wedge that keeps the boat "planing" like a ski on snow.
Say you have a hull that displaces 4000 pounds. It has a piston engine that weighs 500 pounds in it. The engine develops 150 horsepower. Great. Now replace that engine with a rotary engine that weighs 300 pounds. It develops 150 horsepower. But you have only decreased the displacement by about 2 percent. The same 150 hp has to push a boat up hill out of a hole that is still about 2 tons. If you see that the performance gain is just about negligible then the larger picture becomes clear.
Now lets say you add a second rotary engine drive. Now you have only added 100 pounds over the weight of the piston but doubled the horsepower. Doubled the number of props in the water and increased wetted drag almost insignificantly. Now we're talkin!
 
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Well said 02batsea! I could not have said that better myself!

I'll go on record here as saying that this rotary marine adaptation idea..., at best, is silly! Fun project... but silly!
 
Yes if smaller lighter and more horsepower is the objective, then the winner of that contest far and away is a gas turbine. A small helicopter engine weighing a couple hundred pounds will provide 1200 hp easily. They're a tad noisy.
Google "squirt 2"
 
the ideal behind this project is to try something new that no one has done before.I have always liked the rotary engine even though most do not. It is a very simple engine that is tough and reliable in my opinion and I believe that that if it was put in a boat that it will perform very well. The boat it is going in is a light weight boat it weighs about 1800 pounds with the current 2.3 liter in it. My thoughts are with less weight in it it should plain out faster and easier. this experiment might fail badly but it is something that I am willing to try. If nothing else when it is done I will have a great conversational piece
 
the ideal behind this project is to try something new that no one has done before.
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Follow the link above. That ship has sailed.

My thoughts are with less weight in it it should plain(sic) out faster and easier.
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Not likely, see explanation above. Maybe if you stay on shore and not get in it...But seriously you'll get better results trying different props than by changing engines.

If nothing else when it is done I will have a great conversational piece
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Seems like you do already. BUT, we aren't saying you shouldn't go forward. You'll be inside while everyone else is out boating.
 
o2batsea said:
................. You'll be inside while everyone else is out boating.
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I have to agree! Don't let your summer slip by without doing some ordinary boating!

You have come to this forum with a great attitude, and with a thread title that includes "any ideals", which I take to mean "ideas". This would imply that you are open to hearing what others have to say about your idea...... and that is good. And I fully understand that you want to try something new.

It's been discussed as to the issues of torque, horse power, weight, hydro-propulsion, and drive ratio, etc. One member supports the idea that an adapter can be made. I agree!
Post # 15 clearly offers some great information to you as to what some of the hurdles will be.

As for the great conversation piece, should it fail.........., a meterorite makes a great table centerpiece! Both women and children can enjoy it, as well as your men friends......, and it won't leave you stranded out in the middle of the lake! :D

Campo-iron-meteorite.jpg
 
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I have no intentions for letting summer go bye without getting out on the lake. Thats what the other boat is for. Mr O2batsea (hope I spelled that right) I have been unable to find the link that you have mentioned. I was able to find where a couple of websites for marine rotary engines. I also found out that ebbtide produced a boat in the mid 90's that came with that engine. (Darn I wanted to be the first) people that have posted their views about them have nothing but good things to say. Or at least I have not found anybody saying that they hate them yet. I have decided to go forth with this project and hope I don't just end up with a money pti that does not work. I weighed the expence of this conversion with that of repowering with a larger engine and drive and there is not much of a price difference unless some unforseen exppences show up.
 
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