How far is "too far" when raising motor on transom?

[vaughneric2000]

Hello all:<br /><br />I have a 16 Ft Alumacraft with a '78 55HP Johnson. I recently added a "Dolefin" to the motor to get a little more stern lift. The fin works great; however, as a result, as soon as I put the fin on, I lost a little over 1000 RPMs at WOT. The motor was hitting 5300 Max RPMs at WOT prior to the fin install. After the fin install, Max RPMs at WOT dropped to 4250. <br /><br />Holeshot is not very important to me, and I'd like to get those RPMs back.<br /><br />After reading extensively through posts on this subject (motor raising to regain lost RPMs), it's obvious that I need to raise my motor on the transome. My motor is mounted using its lowest holes. There are three more hole settings that I can use to raise the motor on the transome.<br /><br />I have three questions:<br /><br />1. Should I go ahead and raise the motor to its highest possible setting? <br /><br />and/or, <br /><br />Is there a point where raising it too much would be counterproductive to getting back my RPMs? <br /><br />And if so,<br /><br />How can I tell how far is too far in rasining the motor?<br /><br />Thanks a million,<br /><br />You guys are fantastic! I have yet to receive a single piece of incorrect information.<br /><br />Thank you! Thank you!<br /><br />Eric

 

[ob]

Re: How far is "too far" when raising motor on transom?

Raise the engine until the anti-ventilation plate that the doelfin is mounted on is roughly level with the bottom of the boats hull when engine trimmed down level.This is a good starting point.Basically you'd like to get the engine placed as high as possible without experiencing excessive prop ventilation while executing tight turns at around 2/3 throttle on plane.Only a water test can confirm final placement as each set-up is unique.<br /><br />Doelfins will cause added drag particularly if the engine is not at this optimal height placement.While running trimmed out at wot the doelfin should be mostly exposed skimming the water surface.<br /><br />FYI ,that engine should be turning closer to 5500-5800 at wot.You may find that you'll also need to drop down in prop pitch as well as the engine height adjustment to attain this.What diameter and pitch prop are you currently running?

 

[vaughneric2000]

Re: How far is "too far" when raising motor on transom?

Wow! Thanks for the super quick reply.<br /><br />I'm running 11 3/4 x 17p in a stainless Steel prop. The size is the exact same as the stock prop that came on this motor when I bought it "way back when." I just changed to SS because my aluminum props were getting banged and nicked up pretty badly during the normal course of boating in and around the Chesapeake Bay. I got quite a boost in top end from the SS prop, but that's just a plus. Now, I'm just trying to make sure the motor is operating efficiently.<br /><br />I think I'll take your notation that only a water test will really tell me hat I want to know in the end. But do you think I could try the hightest setting and work downward if I don't get what I'm after? Or should I go up 1 hole at a time and test each one? Any way I could damage the engine if I try the former instead of the latter first?<br /><br />Thanks again,<br />Eric

 

[ob]

Re: How far is "too far" when raising motor on transom?

You can start at any point ,so long as you montitor the results making a few turning maneuvers and if excessive ventilation of the prop does not occur ,then you should gain some rpms.Hard for me to say what hole setting is a good starting point without seeing the present set-up.Some slight ventilation coming out of the hole and executing turns is expected so long as it is slight and you still have some trim travel to correct.This is the point at which the prop and lower end is at its least water friction.<br /><br />If you lost 1000 rpm with the addition of the doelfin,your engine without me seeing it is either mounted considerably too low or the doelfin is installed too far forward.

 

[brother chris]

Re: How far is "too far" when raising motor on transom?

Just to add a note. I re-did my transom last year and the dolefin is right at the bottom of the transom. My boat goes real fast(well not right at this moment, cause I have a fuel problem but), when I turn the boat I do not have a very good turning radius at full throttle. If I crank it all the way to either side at full throttle the dolefins come out of the water a little and you can hear the prop coming out of the water a little bit as well. <br />So don't lift it up right to the transom. I would give it at least 1 inch below the transom.<br />That is just my opinion.

 

[DJ]

Re: How far is "too far" when raising motor on transom?

vaughneric2000, <br /><br />Just because that size prop was "stock" does not mean it is the right one. You are overpropped. That engine should reach AT LEAST 5500.<br /><br />Start going up one hole at a time. Or, if your cav. plate is below the bottom, move it up to the even point for a baseline. Move up from there.<br /><br />You really should have a water pressure guage when you sart getting up there.<br /><br />Do not be disappointed if the fin continues to drag you down. They just do that on some rigs.