Need new prop thinking spitfire

[roughneck1455]

Hi all

I am looking to see what you guys all think. I am currently running a Mercury seapro 25HP on a Lowe Roughneck 1455. The boat weighs 500lbs and with myself 250lbs, wife 120lbs and gear my current prop 9.9X11 3 blade, gets me up on plane fairly quick and about a top speed of 24MPH. Around turns my prop seems to come out of water a bit.

I need a new prop as I hit bottom. I was thinking on going with a 4 blade spitfire prop as a buddy of mine loves his. He is running a different boat but thought I would try it. The guy at the parts counter told me he can't believe they sold me a 9.9X11 pitch prop for that boat and motor. He suggests a 10.3X13 pitch if I go with the spitfire.

Do you guys think the boat will run good with a 13 pitch? I have no idea what the RPM is as it is a tiller model but it does seem to run a bit high and loud.

 

[jestor68]

Re: Need new prop thinking spitfire

You need a tachometer to determine present engine rpm before trying any prop.

For your application, suggest a "Tiny Tach". $47; one wire hook up; can be mounted right on the motor cover.

Since the Mercury prop selector calls for a 14 pitch(3 blade) for your application, I would say that the counter guy may be on the right frequency with a 13 pitch spitfire. Still need to verify the rpm before selecting a size.

 

[steelespike]

Re: Need new prop thinking spitfire

Seconded. A little ventilation in turns while a nuisance could be an indication that the motor height is close to ideal,
Could also be an indication the prop is inferior.
The tach is key to motor health and best performance.It will help tell how you present setup is working and what prop change may be needed.And the tach will tell if the new setup is working. A gps for speed is important as well.
The Tinytach will work on almost any motor on land and sea,can be hand held or installed.
From your description I don't think a 13 is a good choice but a tach will tell much better than my opinion.

 

[roughneck1455]

Re: Need new prop thinking spitfire

This Tiny Tach your talking about, How does it work by holding it in your hand but also if you connect one wire to the motor cover, how does it work??

I guess I answered my own question. You wrap the antena around the spark plug wire. So I ordered one on ebay from the US. I should receive it within 3 weeks.


thanks!

 

[roughneck1455]

Re: Need new prop thinking spitfire

Hey guys

I received my tiny tack yesterday and went out today. I am not really sure if the information is enough or not but it was a day with a lot of current. I tested the RPM and I was going up river against the current with a top speed of 13 MPH. but at WOT my motor was reading 4200 RPM. I was told that my motors peak RPM should be between 5000 and 5600 rpm. What do you all think?

 

[steelespike]

Re: Need new prop thinking spitfire

If you drift with your gps it should indicate the current. Without the current we can't really tell much.Except based on your original speed
The current must be 10 mph.?
Is it normal for a motor to labor in heavy current?
13 seems like it is barely planing. 13 at 4200 works out to about 33% slip the reduced rpm and the current would make the slip seem high.Isn't there a place you could test without the current?
Are you sure you didn't mess up the plug wire when you wrapped the antenna wire .Are you sure the plug wire is firmly on the plug.

 

[roughneck1455]

Re: Need new prop thinking spitfire

Hi

Ok went out today on the lake and got some better info. Lets start off by saying I have a Lowe 1455 Roughneck that weighs approx 650lbs with motor. Added weight of myself a friend and gear your looking at approx 650 lbs. My current prop is a 9.9 diameter by 11 pitch 3 blade.

Today with trim set perfect, I was getting 25MPH with a RPM of 5250 WOT.

It took 10 seconds to get on plane against the wind and 8-9 seconds with the wind.


My goal is to try and get my RPM's at ideal and in talking to my local boat shop they said the Sea pro runs ideal at 5500-6000 RPM. I find the boat runs nice the way I tested it today but would like to be able to get on plane quicker. I want a spare prop and so i was thinking the Mercury Spitfire 4 blade. Just trying to figure out what pitch. I am thinking going with a 10 pitch but would like to know what the experienced boater would think.

Thanks


Steelespike I did not set the trim with last figures and that is where the difference was.

 

[steelespike]

Re: Need new prop thinking spitfire

I think your tach is low. Using a 2.25 gear ratio and 11" prop,10% slip and 25 mph I get about 6000 rpm.
At 5% slip about 5600.With your load I think 10% is more likely.
Be sure you follow the tach directions carefully.I think that your hole shot is good for the load your pushing
I have seen some good results with the Spitfire.But I think if your rpm is really 6000 I would stick with a 11" pitch.
I think it will help your hole shot and may help maintain your top end better and put the rpm about 5800,5900

 

[roughneck1455]

Re: Need new prop thinking spitfire

Well if I go by what the instructions say, it says that most single cylinder engines idle at 1400RPM. If it is way high, set it lower and if it is way low, set it higher. My engine runs at 1 spark per revelation but when I put it at the setting it calls for it idles at like 2200 and WOT was close to 9000 RPM. So I put it at the next setting lower. the lower setting idles at 1150RPM.

 

[steelespike]

Re: Need new prop thinking spitfire

Your motor is a twin and I think the idle in gear is about 700-750.
It may be picking up stray pulse. Put the antenna on the plug that allows you to get it away from any wiring
and outside the cover in the shortest distance.
I wonder if your motor has "wasted spark ignition" Would seem to be indicated by the doubling of the apparent rpm.

 

[roughneck1455]

Re: Need new prop thinking spitfire

Ok I will try that. I went to get a spare plug today and noticed that the plugs that came with it are different then what it calls for in the manual so I picked up the right ones. Not sure if it makes a difference or not. I had the antenna around the top plug and will try the bottom one.

Won't be for a couple weeks before I get out again though. Will post an update as soon as I can

 

[roughneck1455]

Re: Need new prop thinking spitfire

I called tiny tach.com and asked them for some help and it turns out that I should have picked up a filtered tiny tach. I guess mine is a regular one. He did say it may still work. He said to rap both plug wires with the least amount of raps and put it on the one setting below the one it should be on. I guess I will try that. Going out Sunday or Monday.

 

[steelespike]

Re: Need new prop thinking spitfire

Did they say why you needed a filtered tach? I haven't heard that previously.

 

[roughneck1455]

Re: Need new prop thinking spitfire

He said for outboards it is best.

BTW you said your motor revs almost 6000 WOT. what do you have for a boat and do you have similar weight in it? Also is it a sea pro or a regular mercury

 

[roughneck1455]

Re: Need new prop thinking spitfire

Well went out again today and here is the info I got with the help from tiny tach.com

At the setting 360 degrees I was getting a speed of 24.4 MPH and a RPM of 10100. So I went to the next lowest setting like they told me and at the 180 degree setting against the wind (15klm) Idle at 1240 WOT 5130 and 23.5MPH

With the wind at 24.5 MPH I got a RPM of 5240 and these figures are me and my GF in the boat.

Now alone in the boat against the wind at 25.2 MPH and 5200 RPM

With the wind I got 26.5 MPH with a RPM of 5330


So with these figures, I am thinking of going with a 9.3X9.5 Mercury Spitfire prop. What do you think?

 

[steelespike]

Re: Need new prop thinking spitfire

OK based on your performance and some research ot appears your motor has a 1.93 gear ratio.
your full load weight is about 1200.What was the weight when tested you alone?
The thing that bugs me is a 9.5 pitch is close to what you would use to push a pontoon.
A 9" pitch is the smallest available here at iboats. Your "fairly quick hole shot" would usually indicate the prop is close to right and maybe the rpm should be higher I guess that depends on that test weight with you alone.

 

[roughneck1455]

Re: Need new prop thinking spitfire

Actually that is something I forgot to tell you. The Gear ratio is 1.92 on this motor. so looks like your pretty close on your figures.

The weight the boat was when I was alone was 160lbs less. On the most part though I have more rather than less weight in the boat. I very rarely go alone. I am going to keep that prop as well. I am going to call to make sure it don't come in a 10 pitch as I thought on the site it said it did. The dealer where I called said it didn't. He said that if I go 11 pitch it will drop the rpm too much. He said general rule of thumb is when you go from a 3 blade to a 4 blade you drop one inch of pitch. He is the one that recommended going with the 9.5 pitch

I have to say I was happy with the performance in yesterday's numbers but when loaded with a couple buddies and I increase the weight in the boat, the performance drops and drives me crazy. I am almost thinking of just having the 2 props and change it according to the weight I have going out.