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Hello all. I've finished a year long restoration of a 1980 Montauk 17. I was hoping to get a few of the experts to give me their input on engine mounting height and propeller selection. The boat has a 1998 Yamaha 115 2-stroke motor that is turning a 17" x 17 pitch prop. As to mounting height, everyone references which mounting holes are used, but my motor is mounted with new holes that don't correspond to the originals. I built a jig to measure the height of the ventilation plate in reference to the bottom of the hull. With the motor trimmed up 3 units so that the ventilation plate is parallel to the bottom of the hull, the ventilation plate is 1.5 inches above the hull reference line.

The boat runs well, but my concern is that it may be under propped. At WOT with full fuel and two people, I'm seeing 45mph at 5900 rpm. This speed is achieved with the motor trimmed up to about 3.5 units which is just prior to the prop ventilating.
According to the Yamaha maintenance manual, the motor is rated at 115hp @ 5000rpm. It also shows a "Full throttle speed range of 4500~5500".

I'd appreciate any input. In reading through old threads from Joe Kriz, Phil T, Finnegan, and Tom W. Clark, I believe that the motor is mounted at the correct height (1.5" above the bottom of the hull). The motor is mounted in the top holes so it could be raised, but not lowered. My bigger concern is with the propeller.

Thank you all,

Jim

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There have been millions of Montauk with motors mounted using the Old School measuring. Not many people measure the exact same way so we would get 20 different measurements for the same boat.

These days we use the bolt holes the motor bracket is mounted in so there is NO need to measure any longer.

In the old days, the motor was mounted all the way down.
Today, for a Montauk 17', you would want to mount it 2 holes up to start with for most motor brands.
https://www.whalercentral.com/article...cle_id=106

Also, for your 1980 model, you probably would need to drill 2 new "Lower" holes unless someone already has replaced the blind holes and drilled these "Green" holes.
https://www.whalercentral.com/article...icle_id=82

Here us a project that changed the bolt holes on the transom of a 15' model but it would be the same for a 17' model like yours.
https://www.whalercentral.com/userpho...lbum_id=64

Let us know how things work out for you and Welcome Aboard Whaler Central.

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Hi Joe. The reason I didn't primarily reference my motors mounting position in relation to the standard hole placement is because the motor is mounted with new holes that do not correspond to your bolt mounting diagram. In my boat, the upper holes are 1.25" down from the top of the transom, and the bottom bolts are 9.5" down from the top of the transom.

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Are those 9 1/2" bottom holes using Lag Bolts?

I do not see how it could be any other way to clear the splashwell with those lower bolts unless they were lags.

Some models can use the "Yellow" holes but usually need the washer bent or ground off like in the photos.

Please tell me they aren't Lag bolts.

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Hi Joe. The lower holes are not lag bolts. They are through-bolted into the splash well at about the same height as the "yellow " holes.

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Alright Jim.

Basically your motor then is already mounted almost 1 hole up due to the top bolts being at 1 1/4" instead of 1 7/8".

So, if it were me, I would go up one more hole and then it would be one hole up in your bracket but basically 2 holes up from the standard mounting holes.

Any photos of your installation?
Always like to see variations that might work for others.

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Remember - selecting a prop is not all about going fast, rather ensuring the engine is operating properly at all throttle settings.

I can say this, the prop is not appropriate. I would expect WOT speed should be at least 50 mph for that engine mode and the Montauk 17.

Running the WOT RPM, gear ratio, slip and target speed through a prop calculator shows a 19" prop.

With the 2.0 gear ratio, the prop diameter should be ~13 to 13.25.

A decent, mid-priced stainless prop includes Yamaha's painted stainless prop in 13x19.

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A 17 would be for the 90hp. You need a 19 or 20" pitch in SS. Vent plate should be 1.5-2" above the keel. Don't run that above 5500.

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This is a photo of the motor trimmed to ~3 units with the ventilation plate parallel with the bottom of the hull. The carbon rod is in line with the bottom of the hull and 1.5" below the ventilation plate.

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Photo did not load but you should be good on height....just need a bigger pitched prop. 2" pitch "usually" equates to a 400rpm drop. You can just have that prop fixed or buy a new or used one and keep that as a spare.

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Photo

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I added photos from OP to the selected posts

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I have a similar vintage Montauk hull (1979), which has the shallow splashwell like yours, and also a 115HP engine. In looking at the photo, I would say the person who mounted that engine did an OK, but barely acceptable job, and certainly not the best. This is a very common problem with installers who don't know Whalers. I could not live with that transom mess shown in the photo.

Considering that is about a 360# engine, I think the top holes are too close to the top. I can also see that the previous engine was probably a Mercury using the old style transom bracket with the thumb screw plates, 1983 and older.

If I were you, and since you have already spent so much time restoring the boat, finish the job correctly. I would remove the engine and fully repair all of the old bolt holes, using wood pegs and polyester resin filler, then gelcoating over. This work will restore the transom strength and make it water tight, besides giving it a new appearance. Note that the outside gelcoat is a different color from the desert tan interior gelcoat.

Then you can redrill holes correctly for mounting the engine, per the drawing and specs here on this site. I would use the 6-1/2" vertical spacing for the bottom hole, which will nicely bring the bolts into the splashwell, and not jammed up on the bottom. This mounting will also give you the 1-1/2" running height which is fine for a 115HP engine on the hull. And because of the heavy engine, I would use a transom stiffener bar across the top set of bolts.

As for the propeller, and because of the potential top speed with the 115, I would use a good SS prop, definitely 19" pitch. From what I can tell, the fastest prop you can buy for a mid-size 4-1/4" gearcase is the new 4 blade Mercury Spitfire-X7. You should pick up 3MPH. Second best choice would be a 3 blade Mercury Laser II. Both come in 19" pitch. These aren't cheap, but you get what you pay for, and they will hold their value for years.
Use a Flo-Torq III hub kit to avoid clutch rattle at low RPM.

I use a Laser II 20" pitch on my Merc in-line 6 "Tower", and it runs about 52MPH @5800 with just me on board. This is scary fast in a Montauk. Most of the time we loaf along at 25-30 MPH.