RPM vs. expected speed

[jason.erbecker]

Running a 46C with 8v71TIs at 2000 rpm I am seeing about 14 knots. Seems a bit slow, but over to the crowd. I’m not sure what wheels are on her other than 4 bladed. I was not loaded down with fuel / water.

 

[SKYCHENEY]

1) Its the bottom and running gear clean?
2) Do you have the right props? (should be in the manual)
3) Do the motors make full rated WOT rpm in neutral?
4) What is top rpm under load?

 

[rustybucket]

Seems very slow to me.

But speed really doesn't matter too much, what is your rpm and speed at WOT when loaded?

 

[jason.erbecker]

1) Its the bottom and running gear clean?
2) Do you have the right props? (should be in the manual)
3) Do the motors make full rated WOT rpm in neutral?
4) What is top rpm under load?

I think the props are in fact the question. Clean bottom and running gear. The RPMs under land get to 2200 and change, not quite 2300 but close enough I think. Perhaps a call to Sams propplace might be in order.

 

[jason.erbecker]

Seems very slow to me.

But speed really doesn't matter too much, what is your rpm and speed at WOT when loaded?

I see about 19-20 knots at 2200ish RPMs. The cruise speed seems slow to me as well.

 

[madhatter1]

I had a 46 Bertram 1973 so assume you have the 435's. That boat cruised at 18 MPH (not knots) at 1950 RPM,s. The Hatteras should be slightly higher speed due to hull shape. Something is wrong.Could be props, dirty bottom/gear, or even under pitched props to compensate for a performance issue. We are "expurts!". You need a real expert.

 

[jason.erbecker]

I found that the wheels I have on right now are 27 x 29s. The boat left Hatteras in 1974 with 28 x 34s. I think I have found the issue, now I just need to find a set of wheels that doesn't put me in the poor house.

 

[Sadey]

Sky, rated RPM in neutral? I know, newb question, but can we do that? I’ve only tried WOT under way but I’m missing 50rpms and I’m trying to track them down.

 

[Pascal]

I found that the wheels I have on right now are 27 x 29s. The boat left Hatteras in 1974 with 28 x 34s. I think I have found the issue, now I just need to find a set of wheels that doesn't put me in the poor house.

If you re turning 2200 and change with these props, it seems to me increasing pitch is only going to make things worst.

How long have you owned the boat? Is this a new problem?

Do the trim tabs work?

 

[SKYCHENEY]

Sky, rated RPM in neutral? I know, newb question, but can we do that? I’ve only tried WOT under way but I’m missing 50rpms and I’m trying to track them down.

Yes, you won't hurt em. Just push them up to WOT in neutral. They should reach the rated rpm on the tag on the engine.

 

[Briankinley2004]

In addition to above do you have the trim tabs down when you are full throttle?

 

[cww]

Just went through this. Skip the props until you have a legit Detroit guy go through your engines. If you’re not turning rated rpm with under-pitched props then the props aren’t the problem the engines are. Once that’s fixed then revisit the props. I just did this in reverse order and wasted 8 grand on props that are the wrong size. Be smart. Don’t be me.

 

[rsmith]

Are you sure you have TI’s and not naturals? 1974 had a few boats had both. Also check your transmission gear ratios. Pre 1980 46 had 1.5/1 after 1980 they went to 1,9/1 because of transmission and shaft failures. I’ve run every variant of the series 1 46 in charter service fishing the offshore canyon in the NE. Pre 1980 with 1,5/1 trans @2100 rpm gives a solid 20 knots for some reason the 1.9/1 boats lost speed and would only run 18.5 knots at 2100. The engines should no load at 2450 and WOT with normal loading @2300. Be advised though that the numbers I saw back in the day were fresh engines and better quality fuel than we get today. If you haven’t been in the engines recently start with a tune up fresh injectors and a compression check.

 

[jason.erbecker]

If you re turning 2200 and change with these props, it seems to me increasing pitch is only going to make things worst. How long have you owned the boat? Is this a new problem?Do the trim tabs work?

The trim tabs work. This is not a new problem. I certainly don’t want to exacerbate the problem with too much pitch.

 

[jason.erbecker]

Just went through this. Skip the props until you have a legit Detroit guy go through your engines. If you’re not turning rated rpm with under-pitched props then the props aren’t the problem the engines are. Once that’s fixed then revisit the props. I just did this in reverse order and wasted 8 grand on props that are the wrong size. Be smart. Don’t be me.

I have a Detroit expert coming in a week or so to go through the engines. Once it’s confirmed that all is well or fixed, I’ll revisit props. Perhaps 28 x 34 is too much pitch. I would imagine a prop shop could put some numbers into their magic black box and recommend a specific diameter and pitch. Thank you for the “be patient” advice.

 

[rsmith]

Make sure he verifies the transmission ratio. They should be Allison M gears it’s easy to do shoot the crankshaft pulley with a digital tach then go back and shoot the shaft coupling. Can be done at idle in the slip with clutch engaged in forward.