340 (496) 8.1 RPM range with props?

[mnm99]

A few years ago I had my props re-pitched because I was above my WOT range of 4200-4600. I was at 4730 so I ended up going from a 18x21 with a .075 cup to a 18x22 with a .075 cup. After that I was at 4620 for some reason and gained like 3mph. So last year I had them go over the props again and add a .125 cup. My speed right now is 28 to 28.5 mph at 3450rpm and a WOT of 38.5 at 4560rpm. I called ACME props and talked to him for a while. He suggested there 18x23 with a .043 cup. He told me it would gain around 1 mph on the top end at 4450rpm and .75 at cruise. Well if you do the math, there is no way i'm spending $3500 for that. So I'm thinking...safe RPM range is 4200-4600. What's stopping me to add another 3/4 or so pitch in my prop. I would drop like 150 rpm bringing me to around 4400 rpm. I should gain better mph at cruise. I know optimal rpm is 4600, but if the range is 4200 to 4600 I should be good? right? Also would this hurt my gas mileage?

 

[Scott Danforth]

boating and fuel economy are a dichotomy

for every HP you command, you burn 0.4# per hour

your currently at 4560 RPM, which is in the window. if you want to go with a bit more pitch, to bring you around 4400, you can do so. however you may cross the balance point and be slower and lug the motor.

I prefer to be under-pitched myself

 

[alldodge]

The standard 496 WOT is 4600-4800 rpm, Rev limiter set at 4950 rpm

Your rpm is getting to low, Was the 3 mph gain a one time thing or every time?

Have a 18 inch prop means B2, and using the info your slip is 20% if you have a 2.0 drive ratio

I would suggest getting your rpm back up closer to 4800. If you could find a B3 lower and 22 pitch props I would try that. Cupping is good for going forward, but backing up causes the props to loose bite

 

[mnm99]

The standard 496 WOT is 4600-4800 rpm, Rev limiter set at 4950 rpm

Your rpm is getting to low, Was the 3 mph gain a one time thing or every time?

Have a 18 inch prop means B2, and using the info your slip is 20% if you have a 2.0 drive ratio

I would suggest getting your rpm back up closer to 4800. If you could find a B3 lower and 22 pitch props I would try that. Cupping is good for going forward, but backing up causes the props to loose bite

Engine spec is 4200-4600 for these motors. I know it's all over the web, but I have the manual that came with the engine and it's also listed on the engine cover. I gained 3mph the first time I re-pitched them. I went from 21 pitch and no cup to 22 pitch and .075 cup. These are inboard V-drives. Not bravo 3.

 

[alldodge]

Engine spec is 4200-4600 for these motors. I know it's all over the web, but I have the manual that came with the engine and it's also listed on the engine cover. I gained 3mph the first time I re-pitched them. I went from 21 pitch and no cup to 22 pitch and .075 cup. These are inboard V-drives. Not bravo 3.

Have a serial number?

 

[mnm99]

Have a serial number?

Sure
0M319042
0M319037
Post the specs when you get them. Thanks

 

[alldodge]

Sure
0M319042
0M319037
Post the specs when you get them. Thanks

Yes my manual shows the same for inboard, sorry my mistake

 

[mnm99]

Yes my manual shows the same for inboard, sorry my mistake

No Problem. I know the whole internet says 4400-4800, not mine for some reason.

So i'm wondering , If I am running at 4550-4580 say now at WOT.. I just noticed my cup is at .250 also. If I take some cup out to say .125 and add 1 pitch to 23, that should bring me to like 4400 WOT. My question is GPH.... Right now I'm running 32GPH at 3450rpm doing 28- 28.5 mph ( I Iike using MPH ) I should gain MPH at cruise, but would I be burning the same GPH? I am trying to get 30 mph if I can. Thanks

 

[alldodge]

Its not going to work out exactly like that. Reducing cup and increase pitch reduces one load and adds another. The more load the more fuel, the less load the less fuel. I would say your best is where you have it about now. The problem with inboard V-drives and (my phrase) tunnel hulls, and just not efficient.

Knew a guy doing the same thing your doing with a 340 Searay Sundancer (twin 454 carb) and flow scan, he finally gave up and just found the best cruise which he liked. He could run it a bit slower and reduce the fuel flow, but it took longer to get there, so more fuel.

I would calculate slip and find least amount and still reaching WOT area.

Just my opinion

 

[mnm99]

Its not going to work out exactly like that. Reducing cup and increase pitch reduces one load and adds another. The more load the more fuel, the less load the less fuel. I would say your best is where you have it about now. The problem with inboard V-drives and (my phrase) tunnel hulls, and just not efficient.

Knew a guy doing the same thing your doing with a 340 Searay Sundancer (twin 454 carb) and flow scan, he finally gave up and just found the best cruise which he liked. He could run it a bit slower and reduce the fuel flow, but it took longer to get there, so more fuel.

I would calculate slip and find least amount and still reaching WOT area.

Just my opinion

I know I am good right now, but there has to be a little more. These engines are animals..I can run 1 person or 10 people with a full tank of gas and it get's on plane and cruises at the same speed every time. I'm telling the truth. People are amazed how this boat runs. I understand about one canceling the other. I was just thinking instead of adding just pitch that would reduce me 200 rpm I could remove a little cup and add pitch. That should bring me down around 125 ish. At this point it's a fine tuning game that cost money...lol. So at the same 3450 rpm you think if I gained even 1 mph I would burn more fuel at the same rpm? I think it's tough to say, because there is so much torque in these engines I may not even see it. Who knows though. I can't find many people tweeking them like me.

I looked at my prop slip and got these numbers
Current =WOT 4580 rpm with 22 pitch prop .1526 slip at 38.5 mph

Proposed =WOT 4400 rpm with 23 pitch prop .1563 slip at 38.5 mph

Current =CRUISE 3450 rpm with 22 pitch prop .1819 slip at 28 mph

Proposed =CRUISE 3450 rpm with 23 pitch prop .2174 slip at 28 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .1895 slip at 29 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .1755 slip at 29.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .1615 slip at 30 mph

 

[mnm99]

Why can't we edit after like 1/2 hr? I wanted to add other MPH readings at 4400 RPM. What would be considered optimal?

I looked at my prop slip and got these numbers

Current =WOT 4580 rpm with 22 pitch prop .15.26 slip at 38.5 mph

Proposed =WOT 4400 rpm with 23 pitch prop .16.73 slip at 38 mph
Proposed =WOT 4400 rpm with 23 pitch prop .15.63 slip at 38.5 mph
Proposed =WOT 4400 rpm with 23 pitch prop .14.54 slip at 39 mph
Proposed =WOT 4400 rpm with 23 pitch prop .13.44 slip at 39.5 mph
Proposed =WOT 4400 rpm with 23 pitch prop .12.35 slip at 38.5 mph

Current =CRUISE 3450 rpm with 22 pitch prop .18.19 slip at 28 mph

Proposed =CRUISE 3450 rpm with 23 pitch prop .21.74 slip at 28 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .20.35 slip at 28.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .18.95 slip at 29 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .17.55 slip at 29.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .16.15 slip at 30 mph

 

[alldodge]

Not fully understanding your numbers, but keeping WOT at 4400 could reduce efficiency. Would rather see rpm in the middle if not the high end.

Since both list 23 may be keeping rpm the same, change pitch and let slip be calculated

 

[mnm99]

Not fully understanding your numbers, but keeping WOT at 4400 could reduce efficiency. Would rather see rpm in the middle if not the high end.

Since both list 23 may be keeping rpm the same, change pitch and let slip be calculated

I kept the cruise the same, but WOT won't be. If I add pitch RPM will drop. I don't know exactly where the MPH will be. The calculator shows this way. That's why I calculated it that way.