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Author Topic:   Engine purchase advice
Barnett Childress posted 07-10-2002 12:24 PM ET (US)   Profile for Barnett Childress   Send Email to Barnett Childress  
Closing in on a deal to repower my Montauk. I'm looking at Yamaha 90 or 70HP two stroke for main and the T8 8hp Hi-thrust 4 stroke for a kicker needed to fish HP restricted water.

Can't decide what would be a better main motor for my needs. Boat used for fishing but also has to pull tubes etc once in a while. Also fairly heavy with bow mounted trolling motor, 3 batteries, Pate 24gal tank. I like the lower gearing on the 70hp (2.33 vs 2.0) and lighter weight (228# vs 261#) Not interested in going 40+ MPH but want to know which motor is best suited to keep me in the mid 30's with heavy loads and a decient hole shot?

Bigshot posted 07-10-2002 02:11 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
If the extra G or less won't kill you....go with the 90. Not much more in performance when running a light rig but will handle the heavy loads better than the 70. My 70 zuki 4 stroke handles heavy loads better than my 90 yamaha did....go figure.
Barnett Childress posted 07-10-2002 07:19 PM ET (US)     Profile for Barnett Childress  Send Email to Barnett Childress     
I think the reason your DF70 does better than your old Yam 90 comes down to more cubes/displacement (79.2 vs 69.6) and lower gearing (2.42 vs 2.0). Less HP loses you a bit of top end.

Seems to me that larger displacement & lower gearing might be bigger factors for good thrust with heavy loads, good hole shot and staying on plane at lower speeds?

Thats why I'm torn between the Yam 90 & 70 motors. The 70 has lower gearing going for it and so should be able to swing a larger prop more easily, but less cubes than the 90 (51.8 vs 69.6).

tabasco posted 07-10-2002 08:53 PM ET (US)     Profile for tabasco  Send Email to tabasco     
As I recall Mercury is changing it's gear ratio on the new 90HP from 2.0 to I think 2.30
whalerron posted 07-10-2002 11:39 PM ET (US)     Profile for whalerron  Send Email to whalerron     
If you can afford the price, go with the 90. You won't be sorry. I have been running my 16' 7" whaler with a Johnson 70hp since 1981 and I just repowered with a new Johnson 90. The new motor is 105 cubic inches and it weighs 319 pounds. The old 70 weighed 202 pounds. I have not noticed any difference in the way the boat sits at rest with the extra 100 pounds hanging on the transom. But, the boat has plenty of power regardless of how much many people are aboard. I do admit though that I haven't tried a hole shot with 30 people on board. I like to run at about 35 mph and that 90 seems to love that speed too.
I think Bigshot is right. Running with just one person on board, you probably aren't going to notice much difference between the 70 and the 90 yammy. But, running with a load is where that 90 is going to shine.

- ron

Jay A posted 07-11-2002 01:40 AM ET (US)     Profile for Jay A    
For what it's worth! When I buy something in particular an expensive item, I use my Quality Control experience (25 years) and drive the sales person nuts!When I bought my Dodge Dakota new,I had them put it up on the lift and checked for fit and finish,leaks and service-ability. When I bought my latest boat I compared hull strength/stability and construction techniques, models such as G/W,Aquasport,Mako,Proline,Seapro and for the engine,fit and finish,I open all access ports and do a visual inspection. And of course,price. In my case I chose a Suzuki 200hp 2 stroke. The visual quality was outstanding and having internal oil capacity which means less can go wrong,really swayed me! Not to mention a better warranty than Yamaha or Merc or Evinrude! So far (knock on wood and there is none in my boat)this second season has been trouble free.
Bigshot posted 07-11-2002 09:37 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Do not compare a 90 OMC to a 90 Yammie. That OMC will crush a Yammie in performance, but you will pay for it in economy. My 70 4 stroke runs 39mph. My 90 Yammie ran 41. A 90 oMC may hit 45.
Barnett Childress posted 07-11-2002 10:42 AM ET (US)     Profile for Barnett Childress  Send Email to Barnett Childress     
Thanks for the replys. The main reason I was looking at Yamaha was light weight for hp. I'm currently running a Merc 90HP 4S and at 386# I feel it is the max weight I would want on the transom.

Actually I'd like to lighten the stern a bit so was looking at trying to stay at a max of 350# for both engines.

So far have a fair deal on Yamaha 70 or 90hp 2 stroke with T8 4S kicker putting total weight at either 328# or 361# respectively.

Best deal and dealership so far has been Merc. 90hp 2 stroke and 8hp 2 stroke kicker. Total weight 382# (where I'm at with single engine now).

Service manager said I could run both 90 injected and premix kicker off of Pate tank by using a good synthetic oil like Amsoil mixed at 100:1. He claims that oil injected 90 will not even notice the difference and kicker rated for 50:1 will run fine on synthetic at 100:1.

This is getting confusing :> Any comments?

Tom W Clark posted 07-11-2002 10:55 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I don't like the idea of mixing at 100:1 for either your main or kicker motors. It's true that a kicker will run fine at 100:1 but there will not be enough oil left behind to protect the cylinder walls between use unless you use the motor all the time. This is why the manufacturers reverted back to their 50:1 recommendation.

It would be much easier to use an inline mixing unit for your kicker. I have one I'll give you if you want. It's left over from when I went from a two stroke to a four stroke on my Outrage. It's an OMC unit but that doesn't matter. It holds about a half gallon of oil and mixes it with the gas from your tank at 50:1 before sending it to the kicker.

Let me know if you want it.

Bigshot posted 07-11-2002 10:56 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Buy a bigger boat:)
Bigshot posted 07-11-2002 10:57 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Tom I have been looking for one of much?
DaveH posted 07-11-2002 01:16 PM ET (US)     Profile for DaveH  Send Email to DaveH     
"Not much more in performance when running a light rig but will handle the heavy loads better than the 70. My 70 zuki 4 stroke handles heavy loads better than my 90 yamaha did....go figure."
The answer to your question is torque. Horsepower is one measure of an engines performance. Without knowing the torque curve of an engine, it's baseless to compare them. It is also helpful to know at what RPM the peaks of both Hp and torque occur as well as their intersection if plotted together. That will help peel away the mystery of why one 90Hp engine is faster or more powerful than another. By no means is this the only measure of performance. There are other very important factors which I'm sure have been addressed to death in the Performance section (e.g. prop pitch, gearing, etc).
Barnett Childress posted 07-12-2002 07:48 AM ET (US)     Profile for Barnett Childress  Send Email to Barnett Childress     
Thanks for the offer but I'll pass for now. Talked to another Yamaha dealer yesterday and he was much more willing to deal & give me a decient trade allowance.

We don't need no stinkin' bigger boat :>! Had a Dauntless 16 before the Montauk. If you see the boats side by side you won't believe how much bigger the Dauntless is. Very happy with the Montauk and wouldn't trade it for anything. Just changing power because of needs/uses.

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