15" vs 20" shaft how do I know which one to use?

[karvin]

Here's a dumb quesiton. As I look to buy a newer motor in the 15HP range I see choices for 15 or 20 inch shafts. I am replacing a Force 15HP motor on an old Gregor 13.4 aluminum boat. I'll guess the 15" model would do. <br /><br />Can someone tell me how this desgination is measured, from what point to what point? I'd like to compare that measure with my Force motor if possible. The Gregor has a fairly low transom on it as well.<br /><br />Thanks,<br />Karvin

 

[Spidybot]

Re: 15" vs 20" shaft how do I know which one to use?

Take a measure from the point where your motor hangs on the transom to the bottom of the transom - at the lowest point.<br /><br />For comparison you could measure the motor from the point where it hangs on the transom to anticavitation plate.<br /><br />Regardles of which points you decide to measure between on your old motor, make sure you check the same on a new.<br /><br />As short/long differs 5" it's easy to tell which your boat is designed for.

 

[rickdb1boat]

Re: 15" vs 20" shaft how do I know which one to use?

 

[karvin]

Re: 15" vs 20" shaft how do I know which one to use?

Thanks Rick! Where did you get that drawing from? That was perfect and what I suspected as well. I'll be measuring tonight.

 

[karvin]

Re: 15" vs 20" shaft how do I know which one to use?

Well I measurd and mine is 16 1/2. So that leaves me in the middle more or less. Would it be better to go with the 20"? Where should the cavity plate be ideally in relation to the bottom of the boat? Obviously below it but by how much?

 

[Spidybot]

Re: 15" vs 20" shaft how do I know which one to use?

Oppinions differ. A slightly too long shaft will be no problem on a boat going modest speeds. On a fast boat the longer (deeper) leg will make the lower unit make too much drag. A prop running close to the surface may break this and call for a propdesign meant for running in a water/air mix.<br /><br />So it depends on how you intend to use your boat.<br /><br />It you're into hi performance you could look for a device/braket that allows moving the motor up and down.<br /><br />Above all it sounds a little weird that a boat designer should make a transom that does not work with standard leg lenghts?

 

[karvin]

Re: 15" vs 20" shaft how do I know which one to use?

Thanks for the info. The boat is an old 1980 Gregor 13' 4" aluminum. I'm not too concerned about speed but certainly want to make sure it planes which it can do now if the kids sit up front and I lean forward.

 

[Spidybot]

Re: 15" vs 20" shaft how do I know which one to use?

Well, you could always get a new with the same length as the old. Would perform the same - and maybe you should opt for the max. rated hp for the boat to make planing more fun/easier.<br /><br />Checking Gregor's homepage it seems as if it's build for 15".

 

[karvin]

Re: 15" vs 20" shaft how do I know which one to use?

Good idea to look on their home page. It seems that the 15" will work fine if mine is the same in terms of the transom. I love this board, everyone is so helpful.

 

[rickdb1boat]

Re: 15" vs 20" shaft how do I know which one to use?

As a general rule, the cavitation plate shoud be even with the bottom of the stern, give or take an inch! Is your 15" able to achive this?