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  Prop for new 4stroke yamy (90hp) on 17 Montauk

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Author Topic:   Prop for new 4stroke yamy (90hp) on 17 Montauk
gazzaba posted 09-15-2005 03:02 PM ET (US)   Profile for gazzaba   Send Email to gazzaba  
I know there are many threads on concerning engine / prop combinations for a 1987 Montauk 17'. However, my question is simple: What prop for a new 4 stroke 90hp yamy? Believe that the merc would be the same.

We purchased the engine and ran it yesterday for the first time. There is so much torque with the prop provided! Due to the the engine responds sensitivly to sensitivly (twitchy) and also tends to pull to the right after up planning and at speed.

I believe this is the result of two issues: prop pitch (please help with what is a prop for this engine), and the directional fin above the prop (please help on how to tune this).

Thanks for your help in advance.

Eddy G posted 09-16-2005 12:39 PM ET (US)     Profile for Eddy G  Send Email to Eddy G     
I repowered my 1994 Montauk with a 90 Yamaha four stroke EFI back in June. I repitched my existing 13" x 22" Laser prop to 20" and it works fine for my use. (Prop shop said they could only reduce the pitch by 2") It turns 5600 rpm with two people on board. Only 5300 with four people. If I was to have bought a new prop, I would have gone with a 13" x 19". I feel I'm right on the edge of being overpropped. You are going to love that engine on your Montauk. The starting, the smoothness, the quietness and amazing fuel economy are well worth the extra expense. I find the added weight makes no difference in the way the boat handles. I've owned my Montauk with the original 90 Mercury two stroke for five years prior to repowering and I can't imagine going back to that. The oil changes are a bit messy if you don't rig up for it. I use a very large funnel with 3/4" hose leading to a container on the ground to catch the oil. Stuff as many rags as you can under the oil filter to catch that spill and it isn't too bad. I'm right at 50 hours on mine and I love it. Enjoy yours.
Eddy G.
SIM posted 09-16-2005 06:55 PM ET (US)     Profile for SIM  Send Email to SIM     
Gazzaba,
You can sometimes counter that torq by turning the anode that is on the underside of your cavitation plate. The bolt that holds it is under the rubber plug that is directly above it. Most of the time this can be done in the water. Run it...adjust it...run it again and so on. Sometimes different trim settings will have different effects so go through the trim ranges while adjusting and testing.

I think a Yamaha 13X19SS or 21al would be a good starting reference point on your 17.

Eddy,

Your F90 should spin near 6000rpms lightly loaded....say by your self. Might be in your best interest and your engines longevity to go with a bit smaller wheel....ecspecially(sp) if 4 people is your normal load. imho

Andy

Eddie Mucciolo posted 09-17-2005 04:49 PM ET (US)     Profile for Eddie Mucciolo  Send Email to Eddie Mucciolo     
Gazz, What type of prop are you using? I have a yamaha prop on my 4S and I will look at the size if you would like. Mine spins 6000 exactly with no unfavorable effects.
Eddy G posted 09-19-2005 09:23 AM ET (US)     Profile for Eddy G  Send Email to Eddy G     
Andy: The Yamaha dealer said my existing Quick Silver tach would work fine on the new 90 four stroke and also the existing prop since the gear ratio is the same as the Classic 90 Mercury. The prop ended up being way to tall at 22" pitch and the tach had not been reset for the new motor. I was very dissapointed in the dealer's installers and their knowledge of that engine. I did not have a photo tach, but only an automotive analizer type tach to check the rpm. I adjusted it, but am not confident in its accuracy. The prop has been repitched to 20". I plan a photo tach test to see if I will have to scrap the prop and/or tach. I agree with you that a 13 x 21 ss is best fit for the 90 4 stroke and the Montauk. By the way, anybody out there know what setting the Quick Silver analog tach would need to be on for the Yamaha 90 four stroke?
Thanks,
Eddy G.
RocketMan posted 09-19-2005 10:39 AM ET (US)     Profile for RocketMan  Send Email to RocketMan     
I know its off topic, and I haven't researched the forum, but are the prop requirements for a four-stroke different than a 2 stroke of the same h.p. ? I guess it could be if the torque curve is signficantly different.
elaelap posted 09-19-2005 06:32 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Plus, RocketMan, the mid-size Yamaha four strokes (and probably the other mid-size four strokes as well) like to run at the high end of their optimum operating range at WOT--that is 5500-6000 rpm--rather than the suggested two stroke 5000-5500 rpm WOT operating range, so perhaps going down a couple of inches in pitch is indicated. Jimh just posted on this subject, verifying some calculations which indicate that a two inch pitch change alters rpm about 450 turns.

I've debated changing my 19" pitch black steel prop, which is stock for the Yamaha 115 two stroke (my last motor), for a 17" pitch prop, since I've now got a four stroke 115 and don't get much over 5500 rpm with the larger pitched prop. My mechanic says to leave well enough alone since the motor has operated flawlessly for 600 hours, but I think I'll try a 17" when the season slows down out here if I can get my hands on one.

Tony

SIM posted 09-22-2005 07:56 PM ET (US)     Profile for SIM  Send Email to SIM     
Tony,

Contact me if you want to try out that demo 17 I have. No strings attached....

Overloaded 4-stroke engines(to low of maximum wot rpm) can lead to higher combustion chamber temps through out all rpm ranges which leads to premature exhaust valve failures. Have you had the valves checked\adjusted? If so did they need re-shimming?

We had a local USCG F225 pair that ran flawless. One day one of the engines lost power and lagged behind the other engine. Did a leak down test and sure enough it had one bad exhaust valve. I did not know at that time they were spinning 19p wheels and could not get over 5600 rpm trimmed up wot. Yamaha covered it under warranty but we got them over to a set of 17's which got the engine in the correct rpm wot range.

Andy

gazzaba posted 09-24-2005 09:40 PM ET (US)     Profile for gazzaba  Send Email to gazzaba     
Engine is running great and quiet, all we wanted.

By the tack, prop turns at 5700 at wot. However, there is still the issue of trim and pull on the wheel. If you trim-up to get the last 200 rpms and a couple of mphs out the wheel pulls hard to the left. trim down and pulls to the right.

goint to try and balance with the little fin above the prop tomorrow so that the boat is trimed out and wheel is pull free at wot.

edfish posted 09-29-2005 01:55 AM ET (US)     Profile for edfish  Send Email to edfish     
Eddy G, I was looking at the shop manual and the tach setting is said to be 6P or 4.

I have twin Merc. 75hp. 4-strokes and think they are great. I have over 2700hrs. on them each and they have had no problems. They purr very nicely at 3000 rpm. with 15" props. My WOT is 4800 rpm. loaded. The range for the 75hp. engines is 4500-5000 rpm.

I fish commercially for halibut, salmon, and crab and have a lot of gear and fuel on the boat. I usually have 30 galllons of fuel onboard but I am not sure why. I only burn 8 to 10 gallons of fuel a day.

The best to all. -ED

Eddy G posted 09-29-2005 09:24 AM ET (US)     Profile for Eddy G  Send Email to Eddy G     
Ed: I appreciate the info. Not sure what it is set on now. I just moved the knob on back of the tach until it was closest to the reading on the analizer/tach I was using. Not very scientific, but all I had to work with at the time. Unfortunately, I haven't been able to use or tinker with the boat since Katrina. The stalls at the marina where I stored the boat blew away in the hurricane. I moved the Whaler to a friend's warehouse here in Baton Rouge before the storm and haven't had a chance to use it since. All of the rivers here in south Louisiana have been closed since Katrina and now Rita. Hopefully we can get a little boating done soon before winter sets in. Thanks again and I'll try those settings with my buddy's photo tach when I get the boat back in the water.
Eddy G.

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