Q1: what is the optimal engine mounting height for a new and yet-to-be-installed Yamaha F90 four-stroke-power-cycle engine on a 1989 MONTAUK 17?
BACKSTORY
My 1989 MONTAUK 17 currently has a similar engine mounted in the lowest possible position. The engine cranking battery has been moved to be inside the center console. There is a 24-gallon on-deck fuel tank under the [helm] seat. There is no auxiliary engine on the transom.
I have read prior discussions about engine mounting height in which mounting one-hole-up or mounting two-holes-up are mentioned.
1989 Montauk 17 Re-power
Re: 1989 Montauk 17 Re-power
Generally mounting the engine at one-hole-up is a good choice, assuming that the propeller to be used can tolerate that slightly higher mounting position and the engine water cooling system does not suffer from reduced flow.
Mounting at two-holes-up may also be feasible, but the chances of propeller ventilation and reduced cooling water flow increase as engine mounting height increases. You are left to decide--perhaps by experimenting--if what (if anything) was gained by the higher engine mounting height was useful and if the effects on ventilation and reduced cooling water flow (should they occur} were not excessively harmful.
The particular propeller design can affect how much higher engine mounting height will affect ventilation. Note that if the propeller has a lot of blade rake, the geometry alone permits higher mounting height because the actual distance below the waterline that the blade tips will be is decreased by the blade rake compared to a propeller with blades with little to no blade rake. This factor alone may be the most significant influence on how well a propeller will resist ventilation at a particularly higher engine mounting height. Also other properties of the propeller such a having cup on the trailing edge will affect how the propeller behaves at a high engine mounting height.
The total boat weight and loading of the boat will also effect how well engine mounting works at a particular height. The difference is only 0.75-inches between "holes", and in a light boat like a MONTAUK the draft of the boat at the stern could easily change by 0.75-inches due to the amount of weight in the boat changing (as with more gear, more fuel, more ice, more people).
Mounting at two-holes-up may also be feasible, but the chances of propeller ventilation and reduced cooling water flow increase as engine mounting height increases. You are left to decide--perhaps by experimenting--if what (if anything) was gained by the higher engine mounting height was useful and if the effects on ventilation and reduced cooling water flow (should they occur} were not excessively harmful.
The particular propeller design can affect how much higher engine mounting height will affect ventilation. Note that if the propeller has a lot of blade rake, the geometry alone permits higher mounting height because the actual distance below the waterline that the blade tips will be is decreased by the blade rake compared to a propeller with blades with little to no blade rake. This factor alone may be the most significant influence on how well a propeller will resist ventilation at a particularly higher engine mounting height. Also other properties of the propeller such a having cup on the trailing edge will affect how the propeller behaves at a high engine mounting height.
The total boat weight and loading of the boat will also effect how well engine mounting works at a particular height. The difference is only 0.75-inches between "holes", and in a light boat like a MONTAUK the draft of the boat at the stern could easily change by 0.75-inches due to the amount of weight in the boat changing (as with more gear, more fuel, more ice, more people).
Re: 1989 Montauk 17 Re-power
I respectfully disagree with Jim.
Two holes up, top bolt in the 3rd hole down from the top.
A good stainless prop is required.
For the Montauk 17 with a Yamaha F90, the owner tested and recommended propellers include:
Two holes up, top bolt in the 3rd hole down from the top.
A good stainless prop is required.
For the Montauk 17 with a Yamaha F90, the owner tested and recommended propellers include:
- 13-1/3 x 18 Yamaha Talon (Performance)
13 x 19 Yamaha Painted SS
13 x 19 Powertech SCD
13.25 x 17” Stiletto Advantage
13-1/8" x 15" Stiletto Advantage Q 4.25
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003
Re: 1989 Montauk 17 Re-power
Phil--I don't see any disagreement. I said that one-hole up is certainly good, and two-holes up may be possible. Where is the disagreement?Phil T wrote:I respectfully disagree with Jim.
Re: 1989 Montauk 17 Re-power
I do not see one-hole as "good."
Two-holes up is, in my experience and research, the minimum. Three holes up is ideal but requires the correct prop and is dependent on use and water conditions.
Note for Mercury owners reading this, you have a 5 hole bracket so the minimum is three-holes up.
:-)
Two-holes up is, in my experience and research, the minimum. Three holes up is ideal but requires the correct prop and is dependent on use and water conditions.
Note for Mercury owners reading this, you have a 5 hole bracket so the minimum is three-holes up.
:-)
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003
Re: 1989 Montauk 17 Re-power
The hierarchy of engine mounting height is:
Lowest possible: unlikely to be the optimum;
One-hole-up: very likely to be an improvement;
Two-holes-up: might be better still, but depends on many factors;
Three-holes-up: suitable for some applications, but not for everyone;
Four-holes-up: racing applications.
EFFECT OF GEOMETRY
The transom of almost all boats is not set at vertical. Usually the transom will have a slight lean aft, perhaps 10-degrees. Also then the boat is on plane, the hull will be trimmed bow high, inducing more angle, perhaps as must as 10-degrees. Assuming the engine trim is set for the propeller shaft to be parallel with the water, this means that the engine mounting bracket is offset from vertical by perhaps 20-degrees.
Because the engine mounting height is controlled by the spacing of the holes on the engine mounting bracket, raising the engine by one set of holes is 0.75-inches in the tilted plane of the transom plus planing angle, but the actual change of the position of the propeller shaft relative to the plane of the water surface will be less, albeit not that much less.
For example: if the transom tilt angle is 15-degrees and the planing angle is 10-degrees, and, again, assuming the engine is trimmed so the propeller shaft is horizontal or parallel to the water, the raising the engine 0.75-inches on the mounting plate only raises the engine vertically 0.68-inches.
EFFECT OF ENGINE WEIGHT
Modern outboard engines generally weigh more than legacy two-stroke-power-cycle engines that were used when many Boston Whaler classic-era boats were designed and built. The increased engine weight is certainly going to increase the draft of the boat at the transom. In terms of propeller performance, the greater engine weight is going to cause the propeller shaft to be farther below the waterline than would occur with a lighter engine. Now we ask: how much more engine weight will cause an increase in draft by 0.75-inches?
The answer can be found if we assume the dimensions of the hull, the shape of the bottom, and the distance the hull is immersed; the density of water is known, so if we can calculate the volume of water that occurs when at the stern the draft is increased by 0.75-inches.
A simple approach to finding how much water volume occur if the draft increases by 0.75-inches is to approximate the hull dimension of beam and length, and to factor the shape that occurs. First we begin with a simple box. The boat beam is generally constant. The length depends on the hull design at the bow. The shape of the hull depends on the bottom design, typically a V-hull or a rounded bottom.
I'll use a classic MONTAUK 17 for the example. We have the beam dimension: 6-feet. For the length, the hull is about 16.5-feet, but let's guess that at the bow there is at least 2-feet-less in the water, so we use 14.5-feet. Now the shape of the hull that is immersed can be considered a rectangle, more or less, for the area from the transom to about 14-feet forward. Farther forward the shape of the immersed hull becomes complex, and for this approximation it will be ignored.
So now we have three dimensions in our box: 14.5 feet x 6-feet x 0.0625-feet (or 0.75-inches). The volume of this water is then 5.44-cubic-feet, and because at this point the further immersion is essentially a box this figure is the volume that will occur. The density of water is about 7.25-lbs-per-cubic-foot, so the added hull weight needed to cause 0.75-inch increase is draft is the about 40-lbs.
This calculation shows that if you increase the engine weight by 40-lbs, the propeller shaft depth in the water is going to increase by 0.75-inches (approximately) on a MONTAUK 17 hull. That means that the engine mounting height could be raised by 0.75-inches to get the propeller shaft back to where it was before the engine weight increased.
These calculations show that the engine mounting height can be changed quite a bit by two factors not considered generally: the angle of the transom and planing angles summed, and the weight increase in the engine.
If anyone reading this has a MONTAUK 17 and wants to verify my calculations, just add 40-lbs right at the transom and measure the increase in draft, and let me know what you find.
Lowest possible: unlikely to be the optimum;
One-hole-up: very likely to be an improvement;
Two-holes-up: might be better still, but depends on many factors;
Three-holes-up: suitable for some applications, but not for everyone;
Four-holes-up: racing applications.
EFFECT OF GEOMETRY
The transom of almost all boats is not set at vertical. Usually the transom will have a slight lean aft, perhaps 10-degrees. Also then the boat is on plane, the hull will be trimmed bow high, inducing more angle, perhaps as must as 10-degrees. Assuming the engine trim is set for the propeller shaft to be parallel with the water, this means that the engine mounting bracket is offset from vertical by perhaps 20-degrees.
Because the engine mounting height is controlled by the spacing of the holes on the engine mounting bracket, raising the engine by one set of holes is 0.75-inches in the tilted plane of the transom plus planing angle, but the actual change of the position of the propeller shaft relative to the plane of the water surface will be less, albeit not that much less.
For example: if the transom tilt angle is 15-degrees and the planing angle is 10-degrees, and, again, assuming the engine is trimmed so the propeller shaft is horizontal or parallel to the water, the raising the engine 0.75-inches on the mounting plate only raises the engine vertically 0.68-inches.
EFFECT OF ENGINE WEIGHT
Modern outboard engines generally weigh more than legacy two-stroke-power-cycle engines that were used when many Boston Whaler classic-era boats were designed and built. The increased engine weight is certainly going to increase the draft of the boat at the transom. In terms of propeller performance, the greater engine weight is going to cause the propeller shaft to be farther below the waterline than would occur with a lighter engine. Now we ask: how much more engine weight will cause an increase in draft by 0.75-inches?
The answer can be found if we assume the dimensions of the hull, the shape of the bottom, and the distance the hull is immersed; the density of water is known, so if we can calculate the volume of water that occurs when at the stern the draft is increased by 0.75-inches.
A simple approach to finding how much water volume occur if the draft increases by 0.75-inches is to approximate the hull dimension of beam and length, and to factor the shape that occurs. First we begin with a simple box. The boat beam is generally constant. The length depends on the hull design at the bow. The shape of the hull depends on the bottom design, typically a V-hull or a rounded bottom.
I'll use a classic MONTAUK 17 for the example. We have the beam dimension: 6-feet. For the length, the hull is about 16.5-feet, but let's guess that at the bow there is at least 2-feet-less in the water, so we use 14.5-feet. Now the shape of the hull that is immersed can be considered a rectangle, more or less, for the area from the transom to about 14-feet forward. Farther forward the shape of the immersed hull becomes complex, and for this approximation it will be ignored.
So now we have three dimensions in our box: 14.5 feet x 6-feet x 0.0625-feet (or 0.75-inches). The volume of this water is then 5.44-cubic-feet, and because at this point the further immersion is essentially a box this figure is the volume that will occur. The density of water is about 7.25-lbs-per-cubic-foot, so the added hull weight needed to cause 0.75-inch increase is draft is the about 40-lbs.
This calculation shows that if you increase the engine weight by 40-lbs, the propeller shaft depth in the water is going to increase by 0.75-inches (approximately) on a MONTAUK 17 hull. That means that the engine mounting height could be raised by 0.75-inches to get the propeller shaft back to where it was before the engine weight increased.
These calculations show that the engine mounting height can be changed quite a bit by two factors not considered generally: the angle of the transom and planing angles summed, and the weight increase in the engine.
If anyone reading this has a MONTAUK 17 and wants to verify my calculations, just add 40-lbs right at the transom and measure the increase in draft, and let me know what you find.
-
msteinkampf
- Posts: 57
- Joined: Tue Nov 15, 2022 4:05 pm
- Location: Alabama/Louisiana
Re: 1989 Montauk 17 Re-power
This is as close as I could come to fulfilling jimh's request for an experimental measurement.
A tape perpendicular to the water was attached to the transom and marked off in inches. The distance from the transom to the water level was initially measured with no added weights. Weights added were two batteries (each 43-lbs each) and about 4.5 gallons of lake water (37-lbs)
Results:
No added weight -------------------------- 11 inches
2 batteries added (86 lbs) --------------- 10 inches (Figure 2)
2 batteries + 4.5 gal. water (123 lbs) -- 9.25 inches
My transom is angled back by about 9 degrees, so for better accuracy the results should be multiplied by 0.988 (cosine of 9 degrees). Looks like your calculations were not far off. These numbers might have been closer to yours if I could have placed the weights right at the transom. Note that the changes aren't linear: 37 lbs. of water gave a depth change that was 75% as much as 86 lbs. of batteries. Perhaps this is because water began entering the splash well drains when the bucket of water was added.
M
- Figure 1. Two batteries, each weighing 43 lbs, and one bucket with 4.5 gallons of lake water. The weights are about 14 inches from the top of the transom.
- BW transom.jpg (63.65 KiB) Viewed 1144 times
A tape perpendicular to the water was attached to the transom and marked off in inches. The distance from the transom to the water level was initially measured with no added weights. Weights added were two batteries (each 43-lbs each) and about 4.5 gallons of lake water (37-lbs)
Results:
No added weight -------------------------- 11 inches
2 batteries added (86 lbs) --------------- 10 inches (Figure 2)
2 batteries + 4.5 gal. water (123 lbs) -- 9.25 inches
- Figure 2. Transom distance after batteries were added.
- BW transom distance 2.jpg (42.62 KiB) Viewed 1144 times
My transom is angled back by about 9 degrees, so for better accuracy the results should be multiplied by 0.988 (cosine of 9 degrees). Looks like your calculations were not far off. These numbers might have been closer to yours if I could have placed the weights right at the transom. Note that the changes aren't linear: 37 lbs. of water gave a depth change that was 75% as much as 86 lbs. of batteries. Perhaps this is because water began entering the splash well drains when the bucket of water was added.
M
1990 Montauk 17’ w 2024 Yamaha F70LA outboard
1995 Aquasport Explorer 245 w twin 1995 Yamaha 150 HP outboards
1995 Aquasport Explorer 245 w twin 1995 Yamaha 150 HP outboards
Re: 1989 Montauk 17 Re-power
Many thanks for taking the time to perform a carefully controlled experiment to assess the variation in rate of change of the freeboard at the transom caused by added weight on the transom. I really appreciate your efforts. I just this moment came across your post, and that caused the delay in my reply.
One other possible cause for the rate of change (in the influence of weight on freeboard) to vary would be the shape of the hull varies as it becomes more deeply immersed. Because of a slight flair to the hull sides and transom, the hull creates more buoyancy as it is pushed deeper into the water by added weight.
Your observation, that at some point of immersion, additional water comes aboard via the (now submerged) engine splashwell drains is a very good assessment. For example, if the weight added becomes extreme, at some point the top of the transom will be below the water line, and a very drastic change in total immersion of the hull will take place, producing a very non-linear outcome.
One other possible cause for the rate of change (in the influence of weight on freeboard) to vary would be the shape of the hull varies as it becomes more deeply immersed. Because of a slight flair to the hull sides and transom, the hull creates more buoyancy as it is pushed deeper into the water by added weight.
Your observation, that at some point of immersion, additional water comes aboard via the (now submerged) engine splashwell drains is a very good assessment. For example, if the weight added becomes extreme, at some point the top of the transom will be below the water line, and a very drastic change in total immersion of the hull will take place, producing a very non-linear outcome.