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Prather Vee 46" with Rossi 61

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Prather Vee 46" with Rossi 61

Old 07-25-2017, 03:20 PM
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Default Prather Vee 46" with Rossi 61

Hello all, I'm having a couple "firsts" here. I've never ran a 46 vee (only 40s), and never a Rossi nitro motor. I don't think getting used to the larger boat will be an issue,if anything it will be easier than the 40 vee bottoms and tunnels I'm used to. I am however a bit lost with the Rossi engine. The needle set up is a large dial with a + and - direction with a needle stem inside. I'm guessing the large dial is maybe idle mix, while the small needle stem in the center of the dial is of course high speed mixture? As far as fuel goes, I'm thinking of a 15% nitro, 20% castor/ synthetic lube for starters, any suggestions? Then a trusty RT5 glow plug from a pile of old NOS Rossis I have. Any comments, suggestions, or ideas are appreciated.
Old 07-26-2017, 02:02 PM
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If you are using 15% i would go with Morgan Omega
Using it for 20 years with as little intnal wear that is possible
The internals stay fresh and new

​​​​​​​Enjoy
Old 07-27-2017, 03:02 AM
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Thank You Tony, I'll have to check and see if my Hobby Store carries that. The stuff I got is Byron, and I have not experimented with fuel past that. I've wondered a lot about fuel though, they stock a huge amount which makes me concerned about freshness. Seems like my engines never really idle that great either, and I've thought it could be a fuel thing? Are there any real concerns about aviation fuel versus fuel labeled for marine use? Seems to me, methanol is methanol, is methanol, and only the mix would be different. I don't do any competitive stuff, just run boats for fun, so I'm not trying to get every last bit of performance. A nice smooth, low idle, with a good transition is what I like. I had an odd thing happen yesterday while I was moving some stuff from boat to boat. I had bought a couple K&Bs to have for spares and spare part motors for my 40" boats when I discovered that one of the older K&Bs was modded, pretty extremely. The ports had been ground and tapered on the outside of the cylinder, and one side of the piston skirt had been cleanly cut all the way up to the wrist pin. I think I'll toss it on an outboard lower unit just to see how these "mods" affect performance. It looks like someone either really knew about timing and such, or didn't have a clue and ruined a jug and piston? I can't really see modding these too much, as it's always been more of a challenge to keep the boat from becoming an airplane, more than engine speed in my limited experience. Really, if a guy thought he needed more power, or RPMs, there's the OS air cooled outboard as well as the Rossi powerhead for the K&B lower unit that clams over 30K RPM. Haven't started the big Rossi yet, but I'm thinking today just to see. It had an open circuit glow pug.
Oh, does anyone happen to know if I'm thinking correctly on the large dial on the carb. being idle mix? Thanks a heap.

Last edited by [email protected]; 07-27-2017 at 03:05 AM.
Old 07-28-2017, 07:10 AM
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Are you sure you only want to use 15% nitro in a 46 sized engine? Most marine engines like at least double to triple that much nitro
Old 07-28-2017, 01:29 PM
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Originally Posted by Hydro Junkie View Post
Are you sure you only want to use 15% nitro in a 46 sized engine? Most marine engines like at least double to triple that much nitro
Not sure of anything with a motor this size, thanks for the response. I was wondering about that, but did not know about doubling. I guess it makes sense with water cooling that you could get away with the extra heat that more nitro will make. Maybe I'll run a couple tanks of this 15% and pick up some 30 next time I get into the city. Looks like I'll get a chance to run this big old thing this weekend. I ordered a new starter belt as the one on it is about shot, but I should be able to get a few starts out of it. It's amazing what 6 inches more boat looks like, this thing is huge compared to my little tunnels and stuff. I'm thinking it's going to be a blast. Hey, these Rossi engines seem to be fairly high dollar, are they that much better than the old K&Bs?

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Old 07-28-2017, 01:30 PM
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Originally Posted by Hydro Junkie View Post
Are you sure you only want to use 15% nitro in a 46 sized engine? Most marine engines like at least double to triple that much nitro
BTW, this is a 61, does that matter when considering nitro percentages in general?
Old 08-01-2017, 10:50 PM
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i wouldn,t waiste money and nitro using anymore than 25 % on a 60 size glow engine. the smaller the engine the more nitro , a 21 loves up to 60% nitro then as we go larger in cc size the percentage drops lower. i remember many years back where i think it may have been John Finch who is a world renown r/c boater with many speed records to his lists , did a review about nitro percentage in larger glow engines such as the 60 size and 90 size and it was mentioned the larger engines did not benefit with the use of high nitro content .
Old 08-02-2017, 10:21 AM
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A more modern current analysis of recommended glow engine fuels...

What fuel is best for your engine? - Model Airplane News

.
Old 08-02-2017, 05:18 PM
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And that article is very true, FOR AN AIRCRAFT ENGINE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
You have to remember, that was posted in MODEL AIRPLANE NEWS, not something like R/C Boat Modeler or the like. If you compare an OS.65AX ABL with a CMB 67 marine engine, you will find something rather interesting:
REQUIREMENTS
Aircraft blend fuel containing 5%-20% Nitromethane with at least 18%
lubricant content.
Propeller: 12x6, 13x6-7 or 14x6
SPECIFICATIONS
Displacement: 0.65cu in (10.6cc)
Bore: 0.95" (24mm)
Stroke: 0.93" (23.5mm)
Weight Without Muffler: 17.5oz (497g)
RPM Range: 2,500-16,000
Output: 1.75 PS, 1.73 HP @ 16,000 RPM You notice the RPM range? 2,500 to 16,000
The CMB 67 runs at 28,000 with no load as compared to the 16,000 of the OS.
This is due to the fact the OS is speed limited by the prop where the CMB isn't. An airplane prop only works when it's running slower than the speed of sound. Too much RPM will have the prop spinning up into the supersonic range, rendering the prop useless. A marine prop, conversely is not speed limited so it can run much faster. This allows the marine engines to use higher nitro than an aircraft engine can to get the higher RPM levels. Obviously, you can get the high RPM without the nitro, this is done in Europe all the time. Here in the States, however, almost all racers run no less than 50% nitro and 18% lubrication. If you have any questions, ask Kirk Maupin what he used to set national NAMBA speed records in Sport 60 and Scale Unlimited Hydroplane
Old 08-02-2017, 06:09 PM
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i would like to add that the o.s. 65 ax is a stock sport engine and the cmb 67 is a full race engine, so you can not compare both of these donkeys sorry.
Old 08-02-2017, 06:52 PM
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And while that is true, one is a sport and the other is a racer, what I said about the OS being prop limited is also true. I don't know of anyone that makes a "sport marine engine", all are full race these days so I took two engines of the same size for comparison purposes
Old 08-08-2017, 09:53 AM
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I want to thank everyone for sharing their hard earned knowledge so freely. What a great bunch of folks I've found here. As a matter of fact, I believe everyone I've met in this hobby have been great people. I read somewhere that for just putting around, 25% nitro, and for serious running 50 to 60%. The Rossi is a little different than the standard K&B I'm used to in that it has a finned head that is also water cooled, so I'm thinking better heat dissipation which allows for higher nitro mixes. I'm trying to get a model boat club started on the little 175 acre lake I live on. Other than something like that, I doubt if there is any real racing that goes on in the Kansas City area. I guess I'll just run the 25% stuff I use in my tunnel hulls since I buy it by the gallon. I still haven't run it. It had a worn out starter belt and I just got some new ones in last week but haven't got it changed out yet. To be honest, I'm a little nervous about running deep vees. I have two 40s and a 46 and haven't had any of them wet yet, ha. I think I might like working on them more than driving them.
Old 08-08-2017, 03:51 PM
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remember this aswell, if you like to use high nitro in your fuel mix you must also change the head clearance or the engine will detonate which will ruin the piston crown and cylinder head button. oh and glowplugs will be ruined .
Old 08-10-2017, 02:08 AM
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Originally Posted by crapshooter View Post
remember this aswell, if you like to use high nitro in your fuel mix you must also change the head clearance or the engine will detonate which will ruin the piston crown and cylinder head button. oh and glowplugs will be ruined .
Do you mean say an extra .010 shim to run 50%, or is there some rule of thumb. It would break my heart to burn up this pretty Rossi motor. I'm going to stick with 25% for now. Heck, that might be plenty fast for me.
Old 08-10-2017, 01:11 PM
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the method of finding the correct head clearance comes from reading the glow plug. example is ; if your glow plugs coil element is squashed in this is an indication of very high compression and some shimming is needed to lower compression. to find the right head clearance for the nitro percentage is a case of blowing / destroying several glow plugs in the process. as you mentioned just stick with 25% nitro and keep the main needle valve slightly on the rich side [ not lean ] your Rossi will perform well and last alot longer. allways keep an eye on the glow plugs condition no matter how much nitro is in the fuel as you could be running the engine lean . [ lean is mean ]

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