Propping Advise

[pyro225]

Hi Guys

I have a propping question... so on paper my boat a Regal 1800 LSR 1.7D 2:1 Alpha One Leg should be achieving 42mph...

However this is what's happening now:

So I have a Solas Prop
14 3/4
21p
WOT 3800rpm / 28MPH

This is not good it meant to do 42 as I say and wot should be 4400rpm.

Now todayI got a Mercury Vengance prop which went on and was tested...
13 3/4
21p
wot 4100rpm but still 28mph...

any ideas? I got it used so cant take it back but have my eye on a laser 2 19p - do you think that would help my top speed and revs issue?​

Thanks

 

[Chris1956]

Dropping the pitch 2 inches will increase RPM by about 400. What makes you think the boat will do 42MPH? A 4.3LX motor should get you close, but a 3.0L motor may be short on power. What have you got?

 

[alldodge]

Either your raising the drive to high, your speedo is incorrect, or your tachometer is off. Here is you slip ratio and your worst then a heavy cruiser.

What are you using for a tach?

 

[pyro225]

Sorry I have a 120hp Cummins Mercruiser Diesel 1.7 4400rpm redline which in most tests (except holeshot) is comparable to the 3.0 als 2:1 drive ratio... just going by what the manual says for top speed ...

 

[pyro225]

sorry meant to say using the onboard tach which i'm pretty certain is good as if you rev it in neutral it revs to 4400. drive is all the way down button wont let it drop more...

 

[Chris1956]

Well, On my 18 foot SeaRay, I needed the 205HP 4.3LX to get to 42 MPH. Not sure your diesel will have the power to achieve that speed. I would guess the 3.0L motor would see 35MPH or thereabouts.

 

[burtonrider11]

sorry meant to say using the onboard tach which i'm pretty certain is good as if you rev it in neutral it revs to 4400. drive is all the way down button wont let it drop more...

You may already be doing this, however, after the boat is on plane, are you trimming the drive out? This should allow the hull to rise a bit thus shortening the running surface, increasing speed and sometimes RPM's of the motor.

The LSR is a stepped hull, so trimming the drive out should really result in some fairly impressive performance. I found one review online that had the same boat but with 4.3l V6 (190hp) topping out at 51.2mph. So with the diesel you should be seeing numbers around 40mph I would think.

The other thing I think you should really check (and others have touched on) is whether your tachometer is truly correct.

 

[pyro225]

Thanks guys - how can I check the tach - if I Rev at idle it goes to 4400 so I'd think it's working?
I can try adjusting the trim can't see it making a load but will try

 

[pyro225]

https://www.ancasta.com/boats-for-sale/regal-1800-lsr-17081/

Shows specs

 

[alldodge]

Thanks guys - how can I check the tach - if I Rev at idle it goes to 4400 so I'd think it's working?
I can try adjusting the trim can't see it making a load but will try

Only way to check the tech is with a handheld or another known good one.

 

[steelespike]

The numbers don't add up.Is the speed by gps?You need to verify the tach as well.
28 mph indicates about 3300 rpm.

 

[pyro225]

Hi yes speed is by gps correct going both ways down river in case of undercurrents as well... So is the tach in neutral going to be different to the tach in gear? As its bang on in neutral as I say

 

[pyro225]

Also if the tach were wrong I'm still nearly 14mph down on spec - I appreciate that might be a light load etc but I'd like to be able to see at least 35ish - my old outboard Glasron ssv151 is only 2ft shorter with a classic 50 does this no stress

 

[alldodge]

Sounds good, but something is just wrong with 25 plus percent slip. The boat is way heavy, the drive is to low, the wrong drive ratio, something isn't right

 

[steelespike]

Your specs indicate that at 28 mph the rpm should be about 3300 rpm. If the tach is accurate then the alternative is very high slip
or perhaps a bad hub in the prop. Bad hub is unlikely as it usually is a total failure. as you increase power the hub will slip at approximately the same range
like a sheared pin.
Only way to confirm the tach is to compare it to a known good source like a shop tach.
Not sure how your diesel gets its tach impulse.
Your rig should perform roughly like a 3.0.
Very likely your power source is compromised possibly as simple as a ineffective throttle.

 

[pyro225]

Hi guys - first of all it's a new hub so it's not that - so next I emailed regal for the official spec which is below - you should see my 1.7 and it's meant to do in excess of 40mph ..z only thing I noticed is it say aluminium 21p prop is stock - mine have all been steel - and after reading you are meant to maybe drop a prop size from aluminium to steel - could this be an issue?

 

[alldodge]

SS can but not always give you a tad bit more speed, but this has no bearing with what were seeing. With a 1.7 drive your slip just gets worst, 3800 rpm = 37 percent and 4100 rpm = 41.6 percent slip.

Are you trimming the drive up as you increase speed?

 

[pyro225]

Hi there no I'm not drive is fully down and when on the plane is nice and level - I will try that but will that equate to 15mph?

 

[alldodge]

Hi there no I'm not drive is fully down and when on the plane is nice and level - I will try that but will that equate to 15mph?

OK, this is your issue. A light went off when you mentioned that your original boat was, and I looked it up and it was an outboard.

An inboard/outdrive (I/O) needs trim to obtain performance and agility.
Start with drive full down. Throttle up and after it gets on plane, start trimming up a little at a time. Keep doing this until it starts to porpoise (nose going up/down), at this point trim down a tad for max speed. The more speed you get the more or less trim you will have. When you go into a hard turn you will learn to trim down a bit so prop does not wash out.

After you do this enough it will be without thinking and you will be trimming up as your coming up on plane. If you trim up to much the prop will ventilate and the rpm's will sky rocket. You will get it

And yes an extra 15 to 20 mph will be found

 

[pyro225]

Ok I will give it a go - which prop of my selection would be best?