Shaft Angle

[dognduck]

Hey every one. What is the deal with shaft angle? I see a couple of people have really harped on changing shaft angle from the stock 15* to closer to 12*. Is it really worth it? Does it really make a difference in speed and efficiency? How about gas vs diesel in shaft angle? I know that the diesel will need a larger diameter prop...

[scenarioL113]

Yes it does make a difference. @12 degrees the thrust is closer to straight out the back which directs it in a more efficient manner.

Maybe a few of the guys on here that actually have done it will chime in. With the better angle some of these 31's are pushing 38 knots PLUS #WOT with 300+ HP engines and cruising 30knots EASY

Keep an eye on Jasons "SNIPE" in the spring when he seatests with his 370's I would guess he will possibly be slightly north of the numbers above...

Like Tony Athens has said..."If you can hold on"...

[Carl]

I put lots of effort into less angle… but 210hp diesels. For me, I’m not diuretic I see an advantage. At my speeds I want my ass out of the water…which more angle provides.

[Tony Meola]

Welcome to the Board.

If I understand Bob Lilco correctly, the trick is when lowering the engine to decrease the shaft angel, you need to keep the strut the same length it currently is and you adjust the angle of the barrel. That allows you to still turn a 20 to 21 inch prop.

Bob's Engine is almost touching the hull.

Reducing the angle actually increases efficiency since the thrust trying to lift the stern is reduced and the boat runs flatter. So more thrust goes into moving the boat forward.

[Carl]

I put lots of effort into less angle… but 210hp diesels. For me, I’m not sure I see an advantage. At my speeds I want my ass out of the water…which more angle provides.

That said…pushing water at an angle closer to direction of travel just makes sense as being more efficient. I’d “almost” like to adjust angle to 15° and take for a spin to compare. Only that adjust would be many days worth of work.

[bob lico]

very difficult to write this post without 10 paragraphs of explanations however tony has explained it the best, you have to understand that for some modifications to work you must do two or three others so they all work in unison. the engines have to be as far back as to have the rear motor mount within 1/4" of fuel tank front support and 1/2 " clearance off the bilge at rear of gears. the propeller shaft coupling is attach the the shaft as one piece no shims and the kevlar shaft log is free to move thru the hull. ( epoxy layup when alignment is completed ) shaft log MUST protrude 6" out the bottom of the hull and front top of struct is cut into the hull (and epoxy 2" backing created from structs to the innovative tides marine F/G rudder supports .using 21" props .tony athens built a 31 Bertram with prop pockets thus ----(hold on) mine you need "no doze" or you will fall asleep at the wheel at 30 knot cruise.like being on a cloud with shock absorbers and 6" cushion seats and a full bridge enclosure.ok now that we have this straight this idea of a 10 degree shaft angle and stupid speed and fuel economy with only work with enough torque to push the boat out of the water enough to ride on the inside strakes AND perfect balance easily known by looking down from bridge at cruise the white water should be breaking directly under the pillars if not you are plowing water and run the danger of the bow "reddening" by wave action and will not attain 30 knot cruise unless you push the motors and fuel economy drops off completely.like if you had CAT 420 hp and you attain 32 knots without the balance you would have the cruise but at big fuel bill.

[bob lico]

you need 2200 lbs aft of the main bulkhead to balance the boat and the reverse chines must be in the water the full length of the cockpit so gas big block mercruiser and cast iron gears are good cummins also good 4 cyl. engine needs 750 lbs of ballast at the transom or a live well fill with salt water to handle 3 to 4" waves at cruise.the enemy of perfect balance is lightweight aft of the main bulkhead in a 31' Bertram

[dognduck]

So aside from thrust direction, less shaft angle also helps getting the hull out of the water when on plane?

[dognduck]

Also, I currently have 260 gas. 1.5 gear ratio and 17D x 15P props with cup. 1&3/8 shafts. 1971 hull. oh and rudders that are the size of credits cards

[bob lico]

to answer dognduck yes once the boat is on plane the boat will have increase in speed or fuel economy however in the case of power plant that have less than 240 hp. or 400 lbs of torque each you will need the original 14 degrees of shaft angle to help the boat get up on plane like a pair of small block mercruisers . trim tabs would also help with minimum power plants . cummins 330/370 would have a output of 1500 lbs of torque much more than needed to put a 31 Bertram up on plane with the heaviest of loads so you can utilized this asset to reduced shaft angle to increase cruise speed by way of increasing the hulls efficiency by way of the boat riding on the inside strakes at about 26-27 knots in addition to no hull surface water friction by way of the entire boat from bow to just under the pillars being entirely out of the water this also helps dramatically in 2 to 4 ' waves .

[DCJ]

Bob, regarding your input on shaft angle back in Nov. 21’, I am currently in process of total resto of 31SF and am repowering original factory gas with 300hp 6BT Cummins with ZF gears. We are at engine mount stage for determining motor mounts and shaft logs as well as ordering shaft struts to accommodate new 1 1/2” shafts so I want to make certain I am perfectly clear as to install. We fish in the Gulf off Texas coast and with the predominant SE wind most days are heading out into pretty much a head sea and the 3-4’ chop can be brutal so smooth/er is paramount. However fuel economy is important as well. My take as of now is go with the 12 degree struts which will still give me enough bow lift to smooth ride and increase economy (ie white water breaking under pillars when viewed from bow) in unison with moving engines back and lowering. What size and type wheels would you recommend? Side bar; I had the pleasure of meeting Cpt. Pat at his place in Llano back when I restored my first 31 back in 2012, what a great man and unbelievable wealth of information. Unfortunately that boat sunk during hurricane Isaac in Venice LA. Had another hull in barn, retired and got bored, so here we go! Have 5 grandsons that love the blue water. Thought it better to leave behind a good boat than cash someday. Thanks for all you guys do!

[Tony Meola]

Bob doesn't monitor the board as much as he used too. Hopefully he picks up on your question, but no matter what you do with the shaft angle you probabley will only be ably to turn a 21 inch prop.

Bob built his props from scratch. He purchased a prop blank that is normally used for the next size up prop then worked it down to the diameter and pitch with the cup he wanted. Doing that gives him more blade area and I believe less slip, creating a more efficent prop.

If you can find a good prop shop, they should be able to help you out. Not sure what the extra cost would be, but asking a prop shop to try is your best starting point.

[bob lico]

again tony has explained the prop i use as well as i could, the blank were a Ron Ellis design his shop was from northern florida. no prop shop is going to put rake in a inboard prop because they don't do that . the best way i could explain the repowering in regards to the boats ability to take head seas is to tell you that you want that oncoming wave to strike the bottom of the hull (keel line) two feet from the bow. this is why you never put a four cylinder engine in a 31 Bertram unless you add ballast at the stern ( you need two thousand pounds aft of the main bulkhead ,not my opinion i experimented and it is imperative to keep the boat bow proud at your lowest cruising speed to WOT. also but keeping the boat bow proud you also keep the last six feet of the reverse chine in the water illuminating "chine walk" and to work in conjunction with the oversize rudders to stabilize the boat in rough water especially with 3' and larger waves and wind off the port or starboard side of the boat at cruise speeds.