Transom Angle and effect in Outboard Trim

[Cajunpockettunnel]

Naah, that is not true. The difference between Al and SS is that the SS blades can be made thinner for a specific blade load, which normally (but not always) is a benefit for a high speed prop, where there is substantial amounts of cavitation and/or ventilation. For the speed range in this case, a good Al screw is just fine. A reasonably competent propsmith can easily add some cup and rake to the existing screws.

Well, I guess the internet is wrong. I read that a long time ago. Something to the effect that Aluminum props have up to 15 or 20 percent slip due to the flexibility. Hell, im not a metallurgist. So, theres that.

 

[Darkzillicon]

You’re not exactly wrong… This is called dynamic pitch loss or prop flex. However, on a 40 hp outboard with the weights involved probably not the factor.

 

[Cajunpockettunnel]

You’re not exactly wrong… This is called dynamic pitch loss or prop flex. However, on a 40 hp outboard with the weights involved probably not the factor.

Ok, then baeckmo is technically right also. I learned something new.

 

[fallguy]

Prop slip and prop flex are the same since when?

 

[Cajunpockettunnel]

Prop slip and prop flex are the same since when?

Doesn't prop flex cause slip? Serious question. I mean i really don't know.

 

[Andrew Leigh]

Had the boat on the water today. On an estuary currently closed to the sea so more fresh water than salt water if that makes a difference to the calc's. Did two runs at 180° to cancel out tide, wind etc.

WOT (Port 5 610rpm, Starboard 5 500rpm)
Avg speed 52.9 km/h (32.9mph)
Hull attitude 1.5°. (Very difficult to measure, phyphox is very sensitive so this is my best guess average)

Must confess to not liking being so close and over the upper limit on the WOT so a pitch / diameter change may not be a bad thing?

 

[Cajunpockettunnel]

Had the boat on the water today. On an estuary currently closed to the sea so more fresh water than salt water if that makes a difference to the calc's. Did two runs at 180° to cancel out tide, wind etc.

WOT (Port 5 610rpm, Starboard 5 500rpm)
Avg speed 52.9 km/h (32.9mph)
Hull attitude 1.5°. (Very difficult to measure, phyphox is very sensitive so this is my best guess average)

Must confess to not liking being so clos and over the upper limit on the WOT so a pitch / diameter change may not be a bad thing?

Most mechanics will tell you that 400 rpm over wont hurt anything. Heck, a mechanic around me claims that 1,000 over won't hurt. And the guy builds racing engines.

 

[Andrew Leigh]

As a matter of interest, I measured and I am circa 75mm lower in the water than the Rigging Manual suggests.

To top line is where the water level actually sits.

 

[Cajunpockettunnel]

As a matter of interest, I measured and I am circa 75mm lower in the water than the Rigging Manual suggests.
To top line is where the water level actually sits.

I think that is ok. I am no expert though. But, with 2 outboards there is a possibility you're stern heavy is all. Someone with more know will pop in with suggestions or ideas.

 

[fallguy]

Stern heavy and bow drop? Does that make intuitive sense to you?

3” too deep, if valid, means the bottom of the engine is creating lotsa drag and forcing the bow down

Consider, for a moment, how a trim tab functions. The bow lifts, the tabs drop down and act like a brake and the bow comes down. An engine too deep is like the tab being down full time.

In order to validate the engine is too deep, take the boat in a turn hard and see if the engines cav when you throttle up; this doesn’t mean turn hard wide open. If they cav, bringing them up will make it worse. A large whirlpool is not the same as a blowout, to be clearer.

 

[Cajunpockettunnel]

Stern heavy and bow drop? Does that make intuitive sense to you?

3” too deep, if valid, means the bottom of the engine is creating lotsa drag and forcing the bow down

Consider, for a moment, how a trim tab functions. The bow lifts, the tabs drop down and act like a brake and the bow comes down. An engine too deep is like the tab being down full time.

In order to validate the engine is too deep, take the boat in a turn hard and see if the engines cav when you throttle up; this doesn’t mean turn hard wide open. If they cav, bringing them up will make it worse. A large whirlpool is not the same as a blowout, to be clearer.

Actually, no it doesn't Fallguy, you are absolutely correct. I forgot about the bow drop. (DUH!)

 

[baeckmo]

As long as the antiventilation plate is in the correct position, there is no extra drag. Note that the instruction is to ensure correct priming conditions for the water pump and avoid excessive resistance to exhaust gases.

 

[Andrew Leigh]

Some feedback on the 110rpm WOT difference. Not sure if I mentioned this fact earlier. Some time ago the starboard prop struck a submerged rock and suffered slight damage to the all the blade tips, this would explain the WOT difference of 110rpm.

If buying the same pitch SS props, would the thinner blade profile manifest in an increase in rpm?

I suppose it is a difficult question to answer as the cupping may well be different.

 

[gonzo]

Prop slip and prop flex are the same since when?

Prop flex is the equivalent to lowering the pitch.

 

[HelmutSheina]

Guys, he trying to get the bow UP. If anything, lower the motors and block them aft a few inches if the harness and cables will stretch. But it probably won't help overall. Getting rid of some forward weight might help.

Of course you can change the transom angle. That's what mounting wedges are for.
Robot or human? https://www.walmart.com/ip/5-Degree-Positive-Tuck-Transom-Wedge-Port-Starboard-1-Set/13932710189?wmlspartner=wlpa&selectedSellerId=101001238&selectedOfferId=598A7A846099349F9277F14DE55F1163&conditionGroupCode=1

As a matter of interest, I measured and I am circa 75mm lower in the water than the Rigging Manual suggests.

To top line is where the water level actually sits.

This ties in with the stalling when trimmed right down at rest. There are holes at the top rear of the trunk below the powerhead to relieve exhaust back pressure in these situations. With the engines sitting so low they can be submerged and cause rough idling and stalling, worse in rougher water.

It is also an engine killer as saline water can enter via exhaust ports and seize pistons and bearings. If the lower spark plugs have greyish milky residue that wipes off with a finger you have water entry into the engine and are heading for worse problems than spray in the face.