CONTINUOUSWAVE

I may make a change to the engine mounting height and the propeller used on a SUZUKI DF140 on a GUARDIAN 19, shown below in Figure 1.



Q1: how much influence on performance does engine mounting height have?

Q2: how much influence does a propeller have on performance?

BACKSTORY
The SUSUKI DF140 engine on my GUARDIAN 19 turning a three-bladed aluminum propeller of 14-inch x 21-pitchc an accelerate to an engine speed of 5,900-RPM and produce a boat speed of 38-MPH.

The boat GUARDIAN 19 boat is located on the west coast [perhaps of the USA], and is not often operated at high boat speeds.

Among information about engine mounting height that I have read, my impression is that a common though is to have the bottom cavitation plate should be level with the bottom of the keel.

The hydrofoil on the engine gear case is the Anti-Ventilation plate. Its purpose is to prevent the ventilation of the propeller.

Cavitation occurs at the blade edges and is NOT prevented by the anti-ventilation plate. Many boaters confuse the terms ventilation and cavitation. For more help in understanding what propeller ventilation and propeller blade cavitation are, read at

https://boattest.com/article/propeller- ... difference

pcloward wrote:...my impression is that a common though is to have the bottom cavitation plate should be level with the bottom of the keel.

Your impression of what is commonly thought is quite different from what is commonly thought on this forum.

On this forum the common thought about engine mounting height is that when the boat is operating at speed and on plane and the engine trim is set so the anti-ventilation plate is level with the water, the anti-ventilation plate should be running just at or just above the flow of water that is traveling across the anti-ventilation plate.

You can see some samples of A-V Plates that are running close to or at or above the flow of water when the boat is on plane at

A-V Plate Photographs
https://continuouswave.com/forum/viewto ... f=7&t=8605

pcloward wrote:Q1: how much influence on performance does engine mounting height have?

Engine mounting height has an effect on boat performance, primarily by reducing the volume of the submerged portion of the engine and therefore reducing the drag induced by that volume. Exactly how to quantify the amount of influence is difficult so only a general answer can be given to your question.

pcloward wrote:Q2: how much influence does a propeller have on performance?

The propeller has enormous influence on the performance of a boat and its outboard engine. No other element of the propulsion system has so much effect--other than the horsepower provided by the engine--at converting the revolution of the propeller shaft into forward or reverse thrust on the hull. Exactly how to quantify or measure the influence is difficult to specify as your question is very general in nature.

pcloward--The anti-ventilation plate should be 1-1/2 to 2-inches above the bottom of the keel.

I appreciate all the information.

I will get some video of the anti-ventilation plate at speed.

Q4: [does “at speed”] means on plane?

Q5: or [does “at speed”] mean at maximum [boat speed]?

When photographing the relationship between the A-V plate and the water flow around the gear case, the boat should be on plane at a typical speed where it cruises nicely, and the boat speed does not have to be the maximum possible boat speed. The engine should be properly trimmed, which means the propeller shaft should be parallel with the water surface, and the A-V plate will then also be parallel to the water surface.

If the gear case and A-V plate are painted white, seeing the plate in foamy water can be difficult, so black tape could temporarily be applied to the upper surface of the A-V plate to enhance the contrast between foamy water and spray and the plate itself.

Trying to take photographs of the A-V plate at very high boat speed could be dangerous.

pcloward wrote:I will get some video of the anti-ventilation plate at speed.

One good photograph is all you need. See the linked article about A-V Plate Photographs above.

Motion pictures are not really necessary, as most of them are just bad "shaky-cam" out-of-focus presentations, but if you are able to get an in-focus and not wildly gyrating motion picture recording, please keep it short, upload the file to youTube, and give the URL. No one wants to watch two minutes of recording in order to see a few good frames showing the A-V Plate location clearly.