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Question. I've recently repowered my montauk with the new 70hp yamaha. Gets up on plan quick enough for me. Don't have speedo on it but feels like it runs out to about 35-40mph.
at top end it started porpusing quite a bit. I trimmed the motor some and it helped but never completely stopped. Would adding dolphin tabs on the foot of the motor help with this?

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Tabs might help but that would be treating the symptom, not solving the problem.

I suspect the motor is mounted too low and you are not using a really good propeller, but please tell us what set of holes the motor is mounted with and what propeller you are using.

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Thanks Tom, I will look tonight and let you know.

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The Yamaha F70 is mounted in the second hole in the top of the mounting bracket. It appears to be approx. one inch above the transom top lip. The motors anti-cavitation plate appears to be level with the bottom of the boat with trim all the way down.

As far as the prop, this is all the markings: It is a PowerTech stainless steel, RED 3R13PYM90, PTPROP.Com 9475325-002-004
With all this, I do not know the prop diameter, the prop pitch, type I guess is PowerTech.

Help! Thanks,

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You have a 13-1/4" x 13" stainless steel three blade PowerTech propeller. Let's go through the numbers:

RED 3R13PYM90, PTPROP.Com 9475325-002-004

RED3 is the model name of this propeller, the 3 indicating it is the three blade version.

13P means it has 13 inches of nominal pitch

YM90 indicates it has an OEM style press-in rubber hub to fit Yamaha outboards with intermediate sized gearcases.

PTPROP.com is PowerTech's web site

9475325-002-004 is the serial number for that particular propeller. PowerTech is unique in that they put a serial number on every propeller they manufacture and they keep a record of this at the factory.

The RED3 is a good prop which PowerTech describes as ..."best all purpose prop, similar to...Stiletto [Advantage]."

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If the bolts go through the second set of holes, the motor is mounted "One Hole Up". If there is about an inch of space between the motor and the transom, this confirms it.

I recommend the motor be raised as high as it can go which will lift it another 1-1/2". The bolts will go through the lowest set of the four sets of bolt holes in the motor. The motor would then be said to be mounted "Three Holes Up".

This alone may eliminate the porpoising. You will gain some speed, fuel economy and have less bow rise on acceleration too. The steering effort will lighter as well.

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Thank you Tom, So what you are telling me is that I need to raise the motor. Currently it's mounted in second from the top of four holes. You are saying I should mount it in the fourth hole-the bottom one? Right?
Won't that but the anti cavitation plate above the bottom of the boat?
Is that right?

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Yes.

Yes.

Yes.

Yes.

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Tom,
Just one more question about this. Most of my boating will be in Mobile Bay that can get rough quickly. Then some river runs and intercoastal waterway travel. Will this be the best set up for me?
Final question, thank you,

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Conventional wisdom used to be that the AV plate needed to be right at the bottom of the hull. It's not so anymore, with modern props. Tom is telling you to raise the motor all the way, as far as it will go without drilling new holes. You may get some grief from a shop that's behind the times, but make them do it (or do it yourself). I think you'll find that it helps a lot with your porpoising situation.

From my experience with the same motor on my 15, I think you have the right prop on your Montauk. With the motor raised you'll probably rev a bit higher at WOT, but that's good - it will improve your accelleration. Your mileage should also improve slightly.

Tim

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Most likely, but you will have to try it to find out.

I do not have personal experience with the PowerTech RED3, but if the prop were the Stiletto Advantage, I am sure the highest motor position would be best.

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See this article under Engines in the article section for mounting height.
http://www.whalercentral.com/articles...cle_id=106

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I live in Mobile and I had your Montauk with a heavier 90hp BRP 90 2 stroke. It was bolted on the lowest hole. Never had issues. Do you keep your batteries and any other heavier items in the stern?

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No the battery is uner the center console, the gas tank uner the seat. so no weight in the stern besides the motor.

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Hiylert,
I am in the process of rigging the same setup, a Yamaha F70 onto a 1988 Montauk. I'll be moving the battery under the console as well. Do you remember the gauge and length of the battery cables you ran to the starter?

Thanks.

Dennis

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Dennis, my F70 came with a set of battery cables. If you get those and they are long enough, I would recommend you use them. They're pretty nice cables, and very flexible which you will need once inside the cowling.

Tim

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If you need new cables contact genuinedealz.com They'll size them and attach the ends to your spec. at an excellent price and turn-around time.
Zap

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It occurs to me to also mention that the duplex (twin conductor) battery cable enters the F70 cowling through a grommet which has cutouts for the various cables - control box, speedo connection, CommandLink bus, battery, and there may be something else I am forgetting. It's an exact fit, that results in a very neat installation - you should expect to do some modifications to the grommet if you use other battery cables.

My recollection is that the motor came with the Yamaha battery cable - I think they were bundled with the motor itself, not the side mount control, but I may be misremembering. The battery cable worked fine on my 15, which routes it around the perimeter of the stern. Depending on where you have your battery located in your Montauk, the stock cable may or may not work, but if it were me I'd check it out before I spent time and money on another set.

Tim

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Tim,
My F70 is mounted on a 15' Sport Center Console at the moment. This has the rigging channel from the console to splashwell. I think it will be too short for the Montauk, but I will certainly check it out first. Both boats are at a remote location at the moment.

Can't wait to get the motor moved. Thanks for the advice!

Dennis

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Sounds like you know the drill, Dennis. I have, from time to time, thought about moving the battery up under the console on my 15, which is a Supersport. But since the Yamaha stock cable works so well, I'd probably go with a heavy cable from the battery to a battery switch in the storage compartment, and then use the stock cable from the switch to the motor. I don't think that'll work in a Montauk - no place to mount the switch "halfway."

Tim

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The ground wire goes directly to the battery wherever the battery is located.
Only the positive wire would go through a switch wherever the switch is located and then from the switch to the battery.

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Quite right, Joe, I'd need some type of safe-at-high-amps) connection for the negative lead. I was thinking of just soldering the old and new negative cables together, covered with heavy, battery-cable specific, adhesive-lined shrinkwrap. Or possibly a post connection, like this one: http://www.defender.com/product.jsp?p...id=1324666

It's likely one of those task I'll never get around to, of course.

Tim

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Tom Clark,
tom, I took your advise and moved the mounting of my new 70 yama 4 stroke in the lowest of four holes raising the motor up approx 1 1/2 inches on my 1988 17 Montauk.
WOW what a difference. It had been mounted one hole up, now it's three holes up. Steering easier, jumps up on plane quick, no apparent cavitation so far. Only had it out for an hour. weather poor. Thanks for the advice.