How fast Should I be Going

[le cras]

I have a Searay Sundancer DA250 With a 260hp V8 305cid with an alpha 1 gen 2 outdrive. Tops out at 4000 revs (checked) and does about 25 knots with a clean hull.
Compression good
Plugs good
Timing good
In fact although it has a standard unit engine and drive they are flawless and perfect in every way.
I think she should go faster but I would like to here from anyone on what they think she should do.
Or even experience of this model.
I have a black max prop but I don’t know about it, and can’t read the part number. It just seams wrong.

Goes up on the plane a bit too easy about 2000 revs with no weed on hull.

 

[skolbe]

Sounds like the boat is propped wrong. Your best bet is to pull the prop and go to a prop shop and share the results. Did the boat ever reach the appropriate WOT range.

 

[Quint4]

Your WOT is a bit low..I think it should be 4600-4800 or so....underproped maybe ? I had the same motor in a 1990 240DA and could get 38mph out of it at 4600 rpm.

 

[DCarl320]

Your WOT is a bit low..I think it should be 4600-4800 or so....underproped maybe ? I had the same motor in a 1990 240DA and could get 38mph out of it at 4600 rpm.

Please explain over vs under propped. Its a little confusing to me.

 

[Weave]

The pitch of the prop impacts the WOT of your boat. I beleive if you drop pitch ( second number on hub of prop) you should increase RPM's and raise pitch you would lower RPM's.
Your case should lower pitch to increase RPM's. That is what is meant by "over propped"

 

[Presentation]

Please explain over vs under propped. Its a little confusing to me.

What are the last two numbers on the prop?

My guess, you should have about 19, or 17 if the prop is cupped.

Put on too high of a number and WOT top end speed goes down and max RPM goes down.

Too low and top end speed goes down and you can over rev the engine.

Goal is at wide open, your tac is about 4600 to 4800 RPM for a 305 V8 small block.

With that boat / motor / drive, you should be able to go 35-36 MPH @ 4,700 RPM.

PS: 35-36 MPH, 30-31 knots.

 

[le cras]

Sorry as i said "I have a black max prop but I don’t know about it, and can’t read the part numbers" 30 Knots was my estimate and mercruisers prop selector agreed. Unfortunatly local agents ar not very helpful i even looked into a stainless steel or high performance alloy. but our only agent wasn't motivated and just fed me a line "thats all it will do" and "A stainless steel prop would wreck the motor in no time"

 

[Presentation]

A stainless steel prop would wreck the motor in no time

Why?

I do not agree with this.

 

[Quint4]

If you "estimated" your speed then this entire discussion is based on conjecture and of limited value.

 

[le cras]

Sorry perhaps I didn’t make myself very clear when I said, “does about 25 knots with a clean hull” I wasn’t guessing my speed I was referring to windage.
This was the major un-recordable factor when averaging the data from many runs over the same-recorded stretch of water.

Measurement was off a pair of logs one tested at our lab, and the third was calculated back from SOG on a GPS and allowance for tide made.

I know life is not exact science but when I’m not able to draw on experience I drop back to it.