170 Montauk Engine Mounting Height Change

[GuardianRC]

I have a 2005 170 Montauk 170 fitted with a Mercury 90-HP FOURSTROKE carburetor engine mounted one-hole-up and running a VENGEANCE 18-pitch propeller. This 170 MONTAUK has the optional stern seat—which I am considering removing—and a Yamaha 4-HP auxiliary engine, so the boat is stern heavy. Also, I usually run with two or three large people on board.

Q1: will raising the Mercury 90 FOURSTROKE engine on my 2005 170 MONTAUK to three-holes-up mounting height be too much height for the way my boat is rigged and used?

Give me recommendations for raising engine mounting height.

ASIDE: I have found only a couple of incidences of a discussion about engine mounting height for a 170 MONTAUK with further raising of the engine. One owner mounted the two-holes higher than original mounting, going to three-holes-up from lowest possible position; that owner reported good [outcomes], however that 170 MONTAUK had a foil appendage fitted to the engine lower unit anti-ventilation plate.

[jimh]

Generally the purpose of raising engine mounting height is to reduce the depth to which the gear case is immersed in the water when the boat is on plane in order to reduce the drag created and in the hope of thereby increasing the potential maximum boat speed that can be obtained at full-throttle when running in very calm water.

If that is your goal, then you can certainly experiment with a further raising of the engine. Otherwise you can leave it at the height is was previously rigged.

Also, if maximum boat speed in calm water is your goal, then you can remove the auxiliary engine and the extra two or three large people. With small boats, every added weight reduces top speed rather significantly.

[GuardianRC]

My goal is to have some of the myriad of improvements which I read for raising engines further than their original positions, at least on the classic hulls. I'm hoping the Montauk 170 responds in the same way. Maybe 1 more hole is a more sensible place to be if there isn't further precedent.

[Phil T]

While the factory mounted the engine 1 hole up, dozens of Montauk 170 I owners have done testing and determined that 2-holes up, top bolt in the third hole counting down from the top is ideal for overall performance.

Removing the bench seat and kicker will improve planing and overall performance as a result of the reduction in stern weight.

The Mercury Vengance is a decent propeller design and an appropriate choice for the engine/boat you have.

I would be interested in your performance after the changes. What is the engine's wide open throttle (WOT) rpm and speed when running solo and light?

[GuardianRC]

Thanks Phil, just the affirmation I'm after. I will get some test figures next time out - whether that is pre or post lifting the engine.

[jimh]

My goal is to have some of the myriad of improvements which I read for raising engines further than their original positions...

There are not a "myriad" of improvements from raising the engine mounting height. The improvement is reduced immersion of the gear case, which reduces drag, and improves ultimate top speed.

There are several detriments to raising the engine mounting height:

• the cooling water intake is closer to the surface and to aerated water coming off the hull, which may reduce cooling capacity for the engine
• the moment arm of the propeller thrust acting on the hull is reduced, so the effect of engine trim on the bow position when running on plane is reduced;
• the propeller blades will be running closer to the water surface and may not work as effectively; and
• in rough seas the propeller may ventilate, which is about the worst thing you want to happen in rough seas; and
• when operating the engine in reverse, stern propulsion may be reduced because more of the propeller trust is hitting the boat transom.

As I said earlier, if your goal is to get the most top speed when running in very calm water at full-throttle, then certainly you should raise the engine mounting height. But the reason engine mounting height at one-hole-up is very common is because it works very well in all situations.

When Boston Whaler fit their 170 MONTAUK with an engine, you would have to expect that they mounted the engine at the optimum height. You seem to think that Boston Whaler had no idea that they could have made a "myriad" of improvements to the performance of their boat if they only had been smart enough to raise the engine mounting height by 0.75-inches more.

[jimh]

Removing the bench seat and kicker will improve planing and overall performance as a result of the reduction in stern weight.

That is true, but it is not a result of raising the engine mounting height.

[GuardianRC]

jimh--The "myriad of improvements" are just reports by other Boston Whaler boat owners on their experience of raising the engines [resulting in] quicker planing, lower speed planing, lower fuel usage, easier steering, higher top speed, more engine trim options, etc. Apologies if the word myriad is too sensationalist for you, however I thought in the context of what could be improved, these reported improvements could be seen as a myriad.

I don't think I have seen a suggestion that a classic Montauk should stick with its 'stock' rigged mounting height, instead advice appears to be at least 2 holes up depending on usage. My other BW, a Dauntless 17DC, porpoises badly with anything but 3 holes up. I know in the past BW did not rig (any?) hulls themselves, but left it to the dealers, but you would think there would be some advice trickling down.

This thread is to ask if there are as frequent reports of the 170 owners having the same positive changes as others see happening on other hulls when raising the engine from the 'stock' mounting height.

I am not looking for ultimate top speed, but the best for overall performance.

PHIL--Thank you for advicethat two-holes-holes up, rather than factory rigged one-hole-up appears to fit the bill.

To surmise I think that Boston Whaler had no idea what they were doing in their research and development in relation to engine mounting height is quite a stretch.

[jimh]

Here is my perspective on your search for reinforcement of your idea about raising the engine mounting height on your 170 MONTAUK by 0.75-inches:

Boston Whaler tested their boat performance and decided they would rig it with a particular propeller and at particular engine mounting height.

You seem to be thinking that Boston Whaler left a lot of improvements unrealized because they did not raise the engine mounting height by 075-inches. You have listed all the improvements you expect to get by your plan to raise the engine mounting height:

1. reduced time to reach plane
2. ability to remain on plane at a lower boat speed
3. reduction in fuel consumption
4. reduction in steering force
5. increase in maximum boat speed
6. increase in engine trim range

This is a stunning list of improvements that are going to occur when the engine is raised.

If all that really is going to happen, you would have to think that the people at Boston Whaler must have no idea what they are doing when they ship their 170 MONTAUK boats with the engine mounted too low. By doing that they are needlessly causing their 170 MONTAUK boats to

1. take longer to reach plane than necessary
2. give up the ability for the hull to plane at lower speeds
3. consume more fuel than necessary
4. require more steering force than necessary
5. miss out on improved maximum boat speed
6. reduce the range of engine trim that is useful

I guess you are just going to have to raise the engine on your 170 MONTAUK by 0.75-inch and get all those benefits, then return to this thread and detail all the measurements before-and-after you made to show how much better the boat runs now that you raised the engine 0.75-inch higher than it was mounted by the factory.

I will be very interested in seeing your test data from before and after the engine mounting height change.

One unique element in your particular boat situation is the boat is heavier in the stern than normal due to the auxiliary engine and the stern seat, so the propeller is probably already farther below the surface of the water than would occur with most 170 MONTAUK boats. When you raise the engine 0.75-inches higher, there probably will not be much change in the tendency for the propeller to ventilate because it won't be as close to the water as would happen on a boat without all the added weight at the stern from your auxiliary engine and stern seat.

ASIDE
One of the problems with changing engine mounting height is the amount of work involved. Once someone raises the engine mounting height, I think there is a tendency to stick with the new position, even if there were no change or even if there were a slight detriment, because returning to the original position will be a lot of work.

[jimh]

Oh, darn, I forgot to mention one more detriment to raising the engine mounting height: performance when making sternway may be reduced because more of the propeller thrust will be hitting the transom. I will emend my list above to reflect the omission.

Also, regarding the meaning of the word myriad, Merrimen-Webster offers this definition:

myriad
noun
1. ten thousand
2. a great number
adjective

1. innumerable
2. having innumerable aspects or elements

Regarding word usage: "Never use a big word when a diminutive one would do."