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Author Topic:   Engine Height
Dave Fahy posted 02-28-2014 06:28 PM ET (US)   Profile for Dave Fahy   Send Email to Dave Fahy  
I have a 99 Conquest 21 with a 2004 Honda 225 mounted one hole up. Should it be two? Is there a measurement that the prop should be from the hull that is best suited for this? It seems low in the water.

Thanks

mkelly posted 03-01-2014 12:27 AM ET (US)     Profile for mkelly  Send Email to mkelly     
I have found that the factory mounting is too low, they typically line up the ventilation plate with the bottom of the transom. You actually want the ventilation plate running slightly out of the water at cruise. You might need to experiment. Pay attention to getting the engine to optimum WOT under normal load, typically around 5,700 to 5,800 RPM. Also, do some cornering to ensure you won't ventilate in turns. Raising the motor is an easy task: lower the trailer jack to the tongue's lowest setting, unbolt the top two transom bolts, loosen the bottom two, lower the motor, and block the skag with plywood or anything adequate. Raise the trailer jack until you're up a bolt hole or two, caulk, and tighten bolts to recommended torque.
Teak Oil posted 03-01-2014 09:56 AM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
Most likely it can be raised. The plate should be noticeably above the water line when on plane.

My 225 on my 22 Outrage could be ran with no problem all the way up, i.e. the bottom set of holes.

I would go up two sets of holes, one set may not be enough to notice a difference. You can't hurt anything, the worst that will happen is if you go to high you may get some cavitation.

Tom W Clark posted 03-01-2014 10:24 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The Honda BF225 has five sets of mounting holes, You are using he second set now so the motor is "One Hole Up".

If your motor had been rigged at the factory in 1999 with a 1999 propeller, and it had hydraulic steering, the motor would have been mounted with the middle set of holes (Two Holes Up). This still would have been a conservative motor mounting height.

You don't say what propeller you are using but if it is a contemporary stainless steel model you can certainly move the motor to the fourth set of holes so the motor is Three Holes Up.

Ultimately, the perfect motor mounting height depend on the propeller used and the way you use the boat and the conditions you operate it in. But Outboard motors have but a few set of mounting holes and they are set 3/4" part so the choices are few.

o
o <-- Bolts are now going through these holes
o
o <-- I recommend bolts through these holes
o

Dave Fahy posted 03-01-2014 12:44 PM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
I appreciate the help. The prop is stainless steel with no visible identifier. The engine always seems low in the water and the boat maxes out at 39 to 40 mph. I am adding 9x12 lenco trim tabs to help with that as well. Picked up the boat in the late fall and the engine always seemed like it was a little too low so this will help. I use it on the south shore of Boston to the Cape and with the fairly large wave action in that area I want to left the engine but not so high it comes out of the water underway.
Tom W Clark posted 03-01-2014 01:33 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
All propeller have identifying marks on them. You may need to remove the propeller from the motor to see the number(s) as they are often cast or stamped into the end of the hub
Tom W Clark posted 03-01-2014 01:37 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
A 21' Conquest with a Honda BF225 should be a LOT faster than 40 MPH. Factory test data I have here shows the 21' Conquest doing in excess fo 45-46 MPH with only 175 HP. With 225 HP I would expect it to go 50 MPH.

95Outrage17 posted 03-01-2014 07:15 PM ET (US)     Profile for 95Outrage17  Send Email to 95Outrage17     
This may help with performance numbers as the 1999 Conquest 21 is basically a 1999 Outrage 21 with a cabin: http://continuouswave.com/ubb/Forum4/HTML/007805.html

- Chris

Dave Fahy posted 03-02-2014 11:19 AM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
I do think it is mounted low so will be interested to test it in the spring. Some other things that could cause it are painted bottom and no tabs. I have a few spring projects that should make it go better. I was initially expecting 45-46 mph so it seemed slow. Looking at 9x12 tabs.
Tom W Clark posted 03-02-2014 11:37 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Yes, rough bottom paint can have a HUGELY detrimental effect on boat speed. Anti-fouling bottom paint that is fair will have very little effect on boat speed.

Trim Tabs serve many important function but top speed is not among them. Adding trim tabs will allow you to level the boat form side to side (but FAR the most important function on a Whaler) and also allow a lower minimum planing speed which can be beneficial is really rough conditions.

Installed properly, trim tabs will have zero effect on top speed. Installed improperly they will reduce top speed.

Reviewing Chris's data, I have to up my top speed expectation for your boat top 52 MPH.

Dave Fahy posted 03-02-2014 05:40 PM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
Seems like a 15 inch 3 blade prop.
Tom W Clark posted 03-02-2014 07:05 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Uh, which one? Can you see a part number? A mark that might indicate diameter? There are several 15" pitch three blades in the Honda propeller catalog and, of course, it could be some other manufacturer's prop too.

How is the propeller's condition? Better yet, do you have a good photo of it?

Dave Fahy posted 03-03-2014 05:47 PM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
I realized once I responded that there are about 7 15" props for Honda. There are no external markings on it. They must be on a hidden part of the prop.
Tom W Clark posted 03-04-2014 01:32 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
There are actually five different stainless steel three blade propeller models in the Honda Propeller catalog plus one aluminum that are available in 15" pitch.

Of the five stainless steel ones I would hope yours, if it is an official Honda accessory, is the 15-1/4" x 15" PowerTech! OFS3, but since that is quite an expensive prop, it is more likely a Solas model. The 15-3/5" x 15" Lexor would be the better choice of the Solas models offered.

Regardless, I recommend you at least try a Mercury Revolution 4 once you raise the motor and clean the bottom.

Dave Fahy posted 03-04-2014 11:55 AM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
Thanks Tom, I look forward to trying these adjustments in the spring. The information exchange on this website is great.
Dave Fahy posted 04-09-2014 06:46 PM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
Does the height of the engine have an effect on rpm or is that a prop only issue. The local shop wanted to look at different props before lifting the engine height. Currently the rpm maxes out about 5000 rpm.
jimh posted 04-10-2014 10:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Raising or lowering the engine mounting height will tend to affect the engine speed at maximum throttle due to changes in the load on the propeller. An engine with higher mounting tends to have the propeller operating closer to the surface or in more aerated water, so the load of turning that propeller may be less compared to the load from that propeller if it were deeply immersed in solid water. The result is more engine speed.

After raising the engine mounting height and getting more engine speed, the propeller pitch will probably be increased. This then increases the load, and drops the engine speed.

The general reason for raising the engine mounting height is to reduce drag by reducing the amount of the gear case and propeller that are immersed in the water. At high boat speeds, it is said that a sizable amount of the total drag on the boat comes from the outboard engine gear case and propeller being dragged through the water, and by reducing the depth of immersion, this drag can be reduced.

Tom W Clark posted 04-10-2014 04:46 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Yes, as jim has explained, the motor mounting height will affect the RPM.

50 to 100 RPM per bolt hole is one rule-of-thumb but it is not a linear relationship and depends on the boat, motor and propeller in question.

Your shop has it backwards. Get the motor height set then experiment with props.

jimh posted 04-10-2014 09:02 PM ET (US)     Profile for jimh  Send Email to jimh     
Also, when the engine mounting height is raised and the drag reduced, that itself reduced the load on the engine. Less load tends to mean more engine speed under load, so there should be some engine speed increase with raised mounting height even without considering the effect of the propeller and what kind of water it runs in.
Dave Fahy posted 04-11-2014 10:40 AM ET (US)     Profile for Dave Fahy  Send Email to Dave Fahy     
I will have them raise it first then. Thanks again.

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