Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  MONTAUK 17 Re-power with Yamaha F70

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   MONTAUK 17 Re-power with Yamaha F70
el Toro 2 posted 08-11-2015 12:27 AM ET (US)   Profile for el Toro 2   Send Email to el Toro 2  
I just put a new Yamaha F70 on my 1988 Montauk. At my dealer's suggestion, I started with a 13-5/8 x 14 propeller. The maiden voyage was today. Followed first-hour break-in of varying speeds under about 2,000-RPM. Then got to the second hour where you are supposed to get boat up on plane. In order to do that with just me and 20-gallons fuel, I had to hit nearly 4,000-RPM and keep [engine speed] there to stay on plane. And I was only going 16 to 17-MPH. Not even half throttle. The boat speed seems way too slow for 4,000-RPM engine speed.

Do I need a different propeller? One with more pitch?

I haven't run at WOT yet because I'm still in break-in, and honestly I'm afraid to. Seems I could redline this thing really easily. To anyone with a new F70 on a classic Montauk 17: what propeller?

tedious posted 08-11-2015 07:57 AM ET (US)     Profile for tedious  Send Email to tedious     
First off, don't worry about hitting the redline - in fact it's better to break in the engine with occasional runs to redline. The F70 has a rev limiter which kicks in at 6450 RPM (vs. the 6300 RPM redline) so you can't really hurt it even if you just bury the throttle (not that I would recommend doing that). If you can take it to 6300 and report on your performance (RPM, speed, trim) that will help people here make a recommendation.

Secondly, can you be more specific about the brand and model prop you are running? That pitch sounds about right, but pitch isn't everything.

Also, if you can give information about your previous engine and performance. That will also help with making a good recommendation.--Tim

el Toro 2 posted 08-11-2015 09:31 AM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
Previous engine was the original 2 stroke Yamaha 90. At 4000 rpms I'd be going nearly 35 mph! I know, apples to oranges, but you asked.

It's a factory aluminum Yamaha prop.

Will report back after WOT operation.

Tom W Clark posted 08-11-2015 10:06 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I recommend the 13-1/4" x 14" Yamaha Performance Series 3-Blade, part # MAR-GYT3B-V4-14

Be sure the engine is mounted high on the transom

jharrell posted 08-11-2015 10:40 AM ET (US)     Profile for jharrell    
I use a Solas New Saturn 13-1/2" x 15" part #3431-135-15 on my 1986 Montauk with the F70 and it works very well. I see 38mph with a light load at 6300 rpm and have even hit 39mph once or twice bouncing off the rev limiter. With a heavier load 36mph is the norm.

I believe 4000rpm is in the mid 20's for me, will have to confirm though. I have my engine my mounted as high as possible and on a jack plate as well, it's actually quite difficult to lose grip or water pressure even when jackplate is raised all the way up.

el Toro 2 posted 08-11-2015 12:57 PM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
Tom W Clark--that propeller is even smaller than what I have. That would be going backwards, right?

jharrell, Yep, I think going up a pitch might help. Your results are exactly what I'm looking for. Although, I'd like to stick with a factory Yamaha prop since my dealer will let me swap out the one I have on it now.

tedious posted 08-11-2015 01:55 PM ET (US)     Profile for tedious  Send Email to tedious     
el Toro--you should probably check out the WOT performance of your current propeller before making any change. Note that by using a cheap aluminum propeller you are leaving performance on the table. Having spent a good chunk of change on your new F70, you might want to consider getting a good stainless steel propeller to maximize performance over the lifespan of your engine.

Can you also clarify: with your previous setup, what was your top speed and RPM; and in what set of mounting holes is your new F70 mounted--Tim

el Toro 2 posted 08-11-2015 02:34 PM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
My previous engine was the original 90-HP two-cycle Yamaha. Top speed was about 43-MPH at about 5,000-RPM. The new engine is currently mounted one hole up. My dealer, who has been installing Yamahas on Whalers for [more than] 30-years, said that is the standard Whaler install. He did say he'd move it up though, if I wanted to.

Please help me understand the effect of mounting engine up or down would have on engine performance. Does moving it higher allow for more RPM?

el Toro 2 posted 08-11-2015 08:47 PM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
Now I'm even more confused about mounting height. I did some research, and I spoke to a prominent West Coast boat builder who just rebuilt the slickest Montauk with an F 70 I've ever seen. [His] logic is to have the engine [Anti-Ventilation or AV] plate even or just slightly above the bottom of hull. With my engine mounted one-hole-up, the [AV-plate] is maybe 0.5-inch above the [keel], which seems perfect. Furthermore, I just walked around the lot at my yacht club where there are no less that seven other classic Montauks. Every single one of them was mounted in a similar fashion. Either one-hole-up or even all-the-way-down in a few cases. None higher than one-hole-up, and lots of different engines.

What is the thought behind mounting three-holes-up? Seems popular here, but I have two trustworthy people with extensive Whaler experience and multiple sample boats telling me otherwise. Please enlighten me.

jharrell posted 08-11-2015 08:51 PM ET (US)     Profile for jharrell    
Generally speaking, moving the engine up reduces drag from the lower unit, among other things, allowing higher [engine speed] and higher top [boat] speed--up to a point.

I have an electric jackplate so I can adjust my engine height on the fly. What I have found is raising the jackplate will continue to increase engine RPM and boat speed at the same throttle-up, until a point when the engine RPM simply goes up with no boat speed increase, denoting loss of grip. If I am going for maximum speed I will raise until it reaches that point. However, this is not ideal in rougher seas as wave action may cause loss of grip sooner. In that case I will put the jackplate down farther.

Without a powered jackplate your will need to find a good compromise for all conditions. Even with my jackplate all the way down it is similar in height as if simply mounted to the hull. So, as recommended I, would mount the engine as high as possible and go down only if you experience unacceptable loss of grip either while accelerating, in turns, or in rough seas.

tedious posted 08-12-2015 07:51 AM ET (US)     Profile for tedious  Send Email to tedious     
When I bought my 15-footer, it came with the engine mounted all the way down. That's the way most boats were rigged 20-years ago, and, as you have found, that is the way many yards will still rig today. The logic you describe is outdated, if in fact it was ever valid at all.

With my 15-footer, after a few trial runs with the engine all the way down, based on advice from this forum, I lifted the engine up one-hole, and then a second-hole or one-hole from all the way up, after I got a good, new propeller. The raised engine was more satisfactory all around: easier steering, less bow rise on takeoff, and reduced porpoising. All of that is very logical if you think about it. The force of the engine is at the center of the propeller, and it tends to push the bow up. When you raise the engine, that force is reduced. And the easier steering is simply due to less gear case in the water, so it's easier to turn the wheel.

Tell your West Coast builder to try lifting the engine. He will be pleasantly surprised, and having been re-educated, will be able to stop parroting that old school advice. And if your dealer is willing, just give it a try; you can always set it back down if you don't like it.--Tim

el Toro 2 posted 08-12-2015 09:09 AM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
OK. I guess [raising the engine is] worth a try. To haul my boat out and take it back and forth to the dealer for experimenting is just a pain. Also, I run this boat in rough Pacific Ocean waters often, I'm a bit reluctant to lose much grip. Would ther be any value to just moving it one more hole up (two up total) as a compromise?
el Toro 2 posted 08-12-2015 09:13 AM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
I also don't want more rpms, which is a reason too for my reluctance to raise the engine. I just want more speed. Is it possible to get more speed without the increase in rpm?
dgoodhue posted 08-12-2015 01:06 PM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
The only way increase the speed with out raising rpm is increasing the prop pitch or cupping. It will be easier to increase the speed with the raised engine. Raise the engine and pick a prop to fit rather than picking a prop to fit inefficient engine height.
el Toro 2 posted 08-12-2015 01:46 PM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
dgoodhue, you may be right. And the more I think about this, the more I'm beginning to see how low mounting height could be contributing to my [boat's lack of performance]. Height first and then propeller selection, the way you explain, seems logical.

The bow rides somewhat high. So I end up having to use more throttle than I'm used to to get it to come down and get up on plane. So, where I should be cruising at 4000-RPM, I'm just barely holding plane. If low engine mounting height creates excessive bow lift, I can see how this is contributing to my problem.

tedious posted 08-12-2015 03:30 PM ET (US)     Profile for tedious  Send Email to tedious     
El T, I did not notice any reduction in grip when raising the engine. Since it matters to you, the best way to ensure a good grip is to install a quality stainless prop.

I would not sweat the RPMs just yet - you need to make that WOT run and see what you get. I am guessing that at 16-17 you were barely planing, just plowing through the water. You need to get over the hump and see what numbers you get.


el Toro 2 posted 08-12-2015 11:40 PM ET (US)     Profile for el Toro 2  Send Email to el Toro 2     
Ok, I put another hour on today and was able to open her up. You know, I think I'm coming around on this [topic of how much engine mounting height should be used with a classic Boston Whaler MONTAUK 17 and a modern engine and propeller]. I feel like I'm pushing thru water. The boat actually feels restricted going thru the water. Bow seem too high. I ran a long straight board from the line of the bottom of the hull and the cavitation plate is too low I think. Unlike other hulls, the Montauk's lowest point is not at the transom. It's a couple feet fore of the transom. So I can now see the wisdom behind mounting it higher. Sorry, I know you all were telling me just that, but I'm one of those stubborn guys who needs to see the logic and figure things out on my own.

The wide-open throttle test was an eye-opener! At best I could only get 5,600 to 5,700-RPM. That was about 29-MPH. Steering feels a little tight. Bow rides high. Seems too slow. All the symptoms of a engine that is mounted too low, right?

Engine speed of 5000-RPM got boat speed of 26-MPH, and 4000-RPM about 18-MPH--that's what it takes to hold plane.

I think my next plan of action is to move the engine up all the way to thee-holes-up. Or, should I only going up one-hole at a time and test again? Then, I want to try the same propeller again with it mounted higher. If not satisfied, I will then go to stainless steel. Or I may eventually still go that route and keep the aluminum one for a spare.

So thank you for your patience all. I feel like I'm starting to get on the right track now. And I realize now, that I was totally wrong. I definitely do need more engine speed. This has been a learning process, to say the least.

tedious posted 08-13-2015 08:31 AM ET (US)     Profile for tedious  Send Email to tedious     
I would recommend moving the engine all the way up, then see how it goes with your current propeller. Based on my experience, you really, really want the WOT engine speed right up at that 6,300-RPM redline.

Your new data is helpful. It calculates out to 9-percent SLIP. Overall, the low maximum RPM, poor top speed, and relatively high slip are indications that your current propeller is all wrong for your boat and engine. But try raising the engine first; it will be interesting to see how much that improves things.


Tom W Clark posted 08-14-2015 10:38 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Tom W Clark, that prop is even smaller than what I have. That would be going backwards, right?

There is a lot more to propeller selection that just diameter and pitch. Be very careful when comparing different models of propeller.

Because the Performance Series 3-Blade will yield calculated slip in the negative 5 percent range, and the aluminum Yamaha prop yields slip in the 5-10 percent range, the same pitch is going to offer very different performance, especially after you raise the engine.

engine is currently mounted one hole up. Per my dealer, who I know for a fact has been installing Yamahas on Whalers for 30+ years, said that is the standard Whaler install. He did say he'd move it up though, if I wanted to.

Yes, that is how engines were typically installed on Whaler 30 years ago, but engienes and propeller have come a vey long way in the last 30 years.

It is very common for dealers and mechanics to install new outboards far too low on Whalers (and other boats) because they are relying on the experience from years and years ago when, in fact, modern stainless steel propeller often allow a engine to be much higher and perform better with less fuel consumption and improved handling.

It is hard to teach an old dog new tricks

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.