Glen-L.com

I am building a 24' 10" Tahoe. I am planning on using a mercruiser 5.7 300HP motor. I was told to use a 14degree strut by Glen L and I got one with an 8" drop. If I have the motor almost as far back as it can go, the propeller ends up about 36" from the transom bottom. On the plans (which are for a 23' boat) the propellor is more like 24" from the transom.

Using the 12" long rudder, it will protrude past the center of the propeller shaft unless I move the rudder farther back.

Question? Is it OK for the propeller to be 3' from the transom and if so, should I move the rudder father back so it's "draft" is about the same as the center of the prop shaft?

Thanks,

Just to make sure you're messuring correctly... I think it is 14 degrees to the motor stringer. That is 4 degree down angle to the motor + 2 degrees keel to stringer + 8 degrees shaft to keel(which I got by an 8 degree down gear) = 14 degrees.

Three feet seems a little far.

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

When I built my boat - I spent a lot of time figuring this out.
It turns out that you can figure out where it goes pretty easily

1. The rudder needs to be completely under the boat.. so - there is very little wiggle room on placement. If it sticks out from under the boat - it will not be as effective. the water is no longer clean once it clears the transom.
2. the prop needs to be spaced forward of the rudder enough so that you can get the prop on & off
3. The center of the prop needs to be such that the blades do not rotate too close to the hull. I seem to remember a formula for that (like 10% of prop diameter or something...) mine is 1.5" I think

4. The prop shaft should stick out the back of the strut by 1x the diameter of the shaft. So the smooth part of the prop shaft on mine sticks out 1" (1" shaft)

with all of that in mind... draw a straight line from prop center to center of output shaft on transmission.

The strut, shaftlog and everything else needs to line up on that line.

You should put the motor exactly where the plans say to put it - it is important to the balance and performance of the boat design. You cant just put it anywhere you like.

I learned all of this by talking to the guys from a prop shop at a boat show.

most of the distances and spacing were formulas based on the prop size, shaft size, etc...

HTH

_________________
Regards,
Bob Perkins

My Current Projects
http://h12restoration.shutterfly.com/
http://nutshellpramconstruction.shutterfly.com/

My Completed project
http://biscayne22.shutterfly.com

Bob is on the money. But, you have a sloped transom. I had to trim the shaft on the rudder to keep it back related to the back bottom edge. The engine should match the plans. But, back is better than forward.

Bill

_________________
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Thanks for the information. After purchasing the recommended prop from Glen-l things didn't seem quite right and I did more research. Based on the boat length, engine size, advice from those with more experienced and talking with prop/shaft dealers, I decided I needed to go with a thicker shaft and more shaft angle. I am now going with a 16 1/2 degre strut, 7 1/2 drop, 1 1/8" shaft.

In order to keep the motor in the same relative horizontal position, I'll need to rise it up (vertically) in relation to the stringers, but I think I have plenty of clearance. This will allow the strut to move farther aft.

The plans show the propeller at about 2' from the transom edge. I stretched the boat 22" so the motor will be about 4-6" farther forward than the plans show and the propeller 2-4". The frame directly under the motor is 6" farther from the transom after the "stretch" so I think I'll be pretty close to the center of gravity if the motor keeps it's relative position to that frame (#4)

Does my logic seem somewhat sound?

THanks, Rick

I'm not too sure what determines shaft OD but I have a BB454 in my boat with a 1 to 1 BW Gear and it's a 1" shaft.

Psycho that sounds small for that power, however my 29hp Kubota ( small block ) has a 1 1/4 " shaft! Go figure.
Doug

lol

Mine isn't a homemade boat, it's in a MFG Mastercraft and that was stock standard for them.

chuckles at slugs small block joke. lol

The inboard engine installation book by Glen l has a lot of info on how to set this up.

_________________
John Amundsen
Monte Carlo
Winter Haven, FL

Maybe it will be done by G6 and maybe not.

Ditto the reference to the inboard installation book by Glen L. It's all there and well laid out.

BTW, the apparent discrepancy in shaft diameters for the BB454 and the 29 hp Kubota can be explained by shaft rpm at rated hp and design safety factor. Higher shaft rpm@rated power gives smaller shaft diam. and higher safety factor gives larger diam. So making a few assumptions (3:1 gear on Kubota, 29 hp@2800 rpm, 1:1 gear on BB454, 350hp@4800rpm, Aqualoy 19 shafts for both), safety factor is about 2.6 for the BB and 11.8 for the Kubota, both within acceptable range. The Kubota shaft would exceed commercial standards and should be bullet proof.

Regards,

Steve

Yes. The most important part is drawing a full scale drawing to check placement.

I'm not crazy about moving the motor up to make the strut right. It raises the center of balance (or gravity)

The full scale drawing will help you determine the strut and angles

(edited to fix stuff I should proof read )

_________________
Regards,
Bob Perkins

My Current Projects
http://h12restoration.shutterfly.com/
http://nutshellpramconstruction.shutterfly.com/

My Completed project
http://biscayne22.shutterfly.com

I'am at the same planing stage with my Tahoe, although I have only built to the plan lenght of 23'. After drawing the rear of the hull out full scale on paper, putting the rudder as far aft as possible and leaving about 4.5" between the rudder and end of shaft, I am going to use a 16 degree strut and a 13" prop this gives me 2" between the edge of the prop and underside of hull, that will put the motor at a 14.5 degree tilt off the stringers. Rick, just double check your interior measurments in the space for the motor, I looked at mercurser too and found that even if I kept it down as tight as I could to the stringers, that I would have to cut the hatch framing down quite a lot in order to get it to fit. Before you buy a motor, look at Indamar, they make a 5.7 litre that has a low profile oil pan, and will sit vary low in the engine compartment.
Bill

THanks for all of the insight. Since I "stretched" the boat I have enough room for a mercruiser 5.7L. I couldn't make it work with a 16.5 degree strut so I'm going with a 14. The end of the shaft will be about 27" from the transom edge. By the time I leave space between the shaft and rudder I think the rudder trailing edge will be about 14" under the boat. I'm trying to find a strut that will take a 1 1/8" shaft (7" drop) but there aren't many place to buy struts on the net.

For Bob Perkins

Congrats on an amazing Herreshoff 12 1/2 restoration project! I've had a set of copies of the original construction drawings from Mystic Seaport for over 10 years. Got the keel poured and then got sidelined. Will be following your progress with great interest.

Regards,

Steve Hansen