calling all prop experts.

[deedubya]

So I have a 20" motor I want to mount on a 25" transom. I could perform some relatively minor surgery ( as transom work goes) and trim down the transom to bring the cavitation plate level with the bottom of the boat. I've checked the air intakes, backwash, etc.. and don't think I have problems if I make the cut. <br /><br />However, I've noticed that you guys talk about raising motors with jack plates and using 4 blade props that are designed to run higher (or slightly out of) the water, for shallow water fishing. Why can't I mount one of these props if they effectively raises the height of the motor, instead of cutting the transom at all? I need to pick up something less than 5". I got a lot more money in the motor (90 hp Mariner) than in the boat (17' capri) and the motor will last longer than this boat, so I don't want to mess with the motor with a shaft extension. Besides, the next hull won't be some quirky 25". Where I live, in small lakes, 20" is 99% of what exists. Thoughts???

 

[The_FoX]

Re: calling all prop experts.

There is no way that you can get another 5" from a prop. Your best bet would be to cut the transom.

 

[deedubya]

Re: calling all prop experts.

Thanks, <br /><br />I did a search while I was waiting, and I quess the type of prop that allows SOME raise in motor height is "heavily cupped". I know I can't get 5" raise, but 2-3 would also help (less cut, less possibility of affecting backwash, etc) So the better question would be, if I'm trying to minimize the depth of the cut in the transom, is there a particular prop that will run shallower than others?

 

[Bob_VT]

Re: calling all prop experts.

Instead of the major boat surgery why not consider getting a lower unit extension. It will be easier and preserve the integrity of the hull. Hpw can you be sure the transom has the integrity to be cut 5"? I could not give my word on it.<br /><br />A 4 plade or 5 blade prop is usually used for heavy loads and rough seas.<br /><br />Bob

 

[JB]

Re: calling all prop experts.

Howdy, DW.<br /><br />I don't like the idea of cutting a transom if there is an alternative. <br /><br />You have two, reversible alternatives:<br /><br />1. As Bob suggests, put a 25" leg on it. How to do that depends on the engine.<br /><br />2. Put a fixed jackplate on it.<br /><br />The designer put a 25" transom on your boat for a reason. Don't second guess him.

 

[Dhadley]

Re: calling all prop experts.

An extension would be the most logical thing to do. However there are a couple of "rule of thumb" things to keep in mind. With jackplates (or set backs) for every 5" you go back (away from the transom) you can go up 1" with the motor. The larger diameter prop you have, the higher you can run it. A 4 blade can (generally) run higher than a 3 blade. A large hydrofoil (like a King fin) allows the motor to be mounted higher. <br /><br />The bottom line is that, yes, you probably can get the 20" motor to work on the 25" transom. One of the things we run into is water pressure. Or lack of it. You may need a low water pick up also. <br /><br />If the boat had a performance hull like a Hydrostream, I would say to go for it. I think you may be better off with your boat to get the extension for the motor.<br /><br />Good luck!

 

[ridiver]

Re: calling all prop experts.

STOP!!!<br /><br />I have a better idea! Do what we did. For about $150 you can get motor bracket with multiple set points. You mount the braket to the intact transom and the motor to the braket. The bracket has about 6 set points so you can dial in motor height. <br /><br />We switched boats and did this to keep our 120 hp motor. I think the bracket was rated to 150 or 175Hp. I don't recall the name of the bracket though.

 

[Dhadley]

Re: calling all prop experts.

ridiver - did that bracket allow you to run a 20" motor on a 25" transom? How much set back does it have? Can you still tilt the motor completely?<br /><br />Let us know!

 

[ridiver]

Re: calling all prop experts.

There is much more than 5" in location options for where the slide plate locks in, so it should allow the 25/20 retro fit. No problems with tilting. Acutally, since the motor is ~6" futher back, it can help tilt problems.

 

[JB]

Re: calling all prop experts.

How is that different than a fixed jack plate? "Fixed" meaning not power adjustable.

 

[ridiver]

Re: calling all prop experts.

JB,<br /><br />The only difference is that this one is very (albeit manually) adjustable up and down. Most jack plates that I have seen have seen are either not this adjustable or include up/down hydraulics. Maybe most of that are that way in which case we are on the same page.