 |
11-20-2020, 11:34 AM
|
#1
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
prop size and clearances
I have been searching for a thread on props sizes and hull clearances .I have a single engine full keel trawler with a 24" prop and only 1.5" clearance. I know I have seen this in another post but can't find it .
also I took out my drive saver until I do a shaft alignment after the boat is splashed . debating on leaving it out. I don't know how moving it an 1.5" toward the keel and away from the rudder will effect the boat. any advice is welcome.
__________________
Advertisement
|
|
|
11-20-2020, 11:58 AM
|
#2
|
Guru
City: Au Gres, MI
Vessel Name: Never Say Never
Vessel Model: President 41 DC
Join Date: Jul 2015
Posts: 10,515
|
Quote:
Originally Posted by timb
I have been searching for a thread on props sizes and hull clearances .I have a single engine full keel trawler with a 24" prop and only 1.5" clearance. I know I have seen this in another post but can't find it .
also I took out my drive saver until I do a shaft alignment after the boat is splashed . debating on leaving it out. I don't know how moving it an 1.5" toward the keel and away from the rudder will effect the boat. any advice is welcome.
|
If I remember correctly you want a minimum of 10% and preferably 15% clearance.
We took our drivesavers out and replaced them with spacers from Spurs. They mad the spacers to fit and they fit perfectly. We didn’t have room to slide the prop shaft forward. You should have more shaft length behind the strut in between the strut and prop than the thickness of the shaft. Out shafts are 2” so our prop should be within 2” of the strut.
__________________
__________________
Boat Nut:
If you are one there is no explanation necessary.
If you aren’t one, there is no explanation possible.
|
|
|
11-20-2020, 12:11 PM
|
#3
|
Guru
City: Cape Cod, MA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 1,162
|
Yes, 10%D is the absolute minimum clearance above w/ 15% being better. 10% below to the skegbar is enough. If you stand on the hull above the prop at speed and feel strong vibration or hear sounds like someone playing a hose on the hull, you may be too close.
I lowered my skeg and shaft on my downeast lobsterboat to add 2" above and 1" below my 22"D prop. This resulted in almost 1k in crusing speed and 1k at WOT plus much less vibration and prop noise w/ no other chnges.
|
|
|
11-20-2020, 12:14 PM
|
#4
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
ok I definitely don't have 10% of dimeter maybe of the radius . looks like I need to do some planning for the next haul out. out of money this year well that type of money.
I do have the over 2" to the bearing but the donut zinc is going to take up a lot of it.
|
|
|
11-20-2020, 02:29 PM
|
#5
|
Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,635
|
Looks to me like you’ve got too much prop.
How much power is in that boat?
__________________
Eric
North Western Washington State USA
|
|
|
11-20-2020, 02:36 PM
|
#6
|
Guru
City: Au Gres, MI
Vessel Name: Never Say Never
Vessel Model: President 41 DC
Join Date: Jul 2015
Posts: 10,515
|
What is the distance from the back of the skeg to the front of the prop. It should not be more than 1 shaft thickness. Out shafts are 2” so I can’t go more than 2” unsupported shaft from the last strut, in your case the skeg, to the front of the prop. Otherwise if it is too long you may break the shaft. Our props are very close to the last strut so I had to have spacers made when I removed the drivesavers. But I was able to align the engine properly and did loose some vibrations.
__________________
Boat Nut:
If you are one there is no explanation necessary.
If you aren’t one, there is no explanation possible.
|
|
|
11-20-2020, 07:58 PM
|
#7
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
It’s exactly the shaft diameter. I guess it never should have had the drive saver . That prop has probably been on the boat 20 years
|
|
|
11-20-2020, 08:01 PM
|
#8
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
It’s a American diesel 136sn or something like that pretty much a 135 Lehman
|
|
|
11-20-2020, 08:08 PM
|
#9
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
I have been following the prop pitch threads and economy ones I only get about 7kts at 1900 I thought a 38 should have a better hull speed than that . I figured I was under proped maybe my power is just beating into the hull and skeg
|
|
|
11-20-2020, 08:47 PM
|
#10
|
Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,635
|
Under propped will probably get you the most speed ... from the most power.
What’s your rated power rpm?
And what’s your WOT rpm?
I’m guessing now but suspect you need to reduce pitch.
__________________
Eric
North Western Washington State USA
|
|
|
11-20-2020, 08:57 PM
|
#11
|
Guru
City: Au Gres, MI
Vessel Name: Never Say Never
Vessel Model: President 41 DC
Join Date: Jul 2015
Posts: 10,515
|
I think that you need to reduce the diameter so you get at least 10% clearance first. Then see what the RPMs are at full throttle. I have SP225s so I am not sure what your max RPMs should be but you want to hit the max. Boatdiesel.com has some calculators as to what size would be a good start. There is a yearly membership fee on boatdiesel so maybe you can find a free calculator somewhere else. Good luck and keep letting us know how it is going.
__________________
Boat Nut:
If you are one there is no explanation necessary.
If you aren’t one, there is no explanation possible.
|
|
|
11-21-2020, 07:49 AM
|
#12
|
Guru
Join Date: Jun 2008
Posts: 4,078
|
Quote:
Originally Posted by timb
I have been following the prop pitch threads and economy ones I only get about 7kts at 1900 I thought a 38 should have a better hull speed than that . I figured I was under proped maybe my power is just beating into the hull and skeg
|
Actually that rpm/speed sounds about right to me.
__________________
Jay Leonard
New Port Richey,Fl
|
|
|
11-21-2020, 02:09 PM
|
#13
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
On the survey it reached 2700 the ratesd rpm is 2600 . I have never pushed it past 2100I just saw the prop markings it’s a 24 L20.
|
|
|
11-21-2020, 02:39 PM
|
#14
|
Guru
City: Miami Florida
Vessel Name: Possum
Vessel Model: Ellis 28
Join Date: Aug 2011
Posts: 4,730
|
My boat Possum had very little clearance between the skeg and the prop tip. The wash off the prop tip actually blasted a shallow dimple into the solid glass skeg.
__________________
Parks Masterson
|
|
|
11-21-2020, 04:26 PM
|
#15
|
Guru
Join Date: Jun 2008
Posts: 4,078
|
Quote:
Originally Posted by timb
On the survey it reached 2700 the ratesd rpm is 2600 . I have never pushed it past 2100I just saw the prop markings it’s a 24 L20.
|
Sounds like that is propped pretty good as long as the tachs are accurate.
__________________
Jay Leonard
New Port Richey,Fl
|
|
|
11-21-2020, 04:58 PM
|
#16
|
Guru
City: Au Gres, MI
Vessel Name: Never Say Never
Vessel Model: President 41 DC
Join Date: Jul 2015
Posts: 10,515
|
Quote:
Originally Posted by HopCar
My boat Possum had very little clearance between the skeg and the prop tip. The wash off the prop tip actually blasted a shallow dimple into the solid glass skeg.
|
That is one of the reasons I think that he should go smaller on the diameter.
__________________
Boat Nut:
If you are one there is no explanation necessary.
If you aren’t one, there is no explanation possible.
|
|
|
11-21-2020, 06:33 PM
|
#17
|
Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,635
|
Actually it may be best to go smaller on the diameter.
Cut it down. Just change the diameter.
It’s almost a square prop so no doubt suffers from friction water to blade.
If it’s a 2.57-1 gear going to the 2.93 (or whatever it is) and increase pitch as required would probably get-her into the good ratio zone.
timb wrote;
“On the survey it reached 2700 the ratesd rpm is 2600 . I have never pushed it past 2100I just saw the prop markings it’s a 24 L20.”
Well I’m say’in run her up there and verify. That’s perfect by the way ... 100rpm over rated.
Don’t change anything till you get the;
1. Rated max power rpm.
2. Check your tach and then go to WOT. Since it concerns you only stay at rated rpm for about a minute. As soon as it stabilizes observe the rpm. Make sure it isn’t still climbing. It won’t hurt your engine if there isn’t anything wrong with it. You can safely go 2 or 300rpm over rated rpm. A govoner in the engine’s injection system to prevent over reving. Not to worry.
Let us know or post before then if you like.
__________________
Eric
North Western Washington State USA
|
|
|
11-22-2020, 01:35 AM
|
#18
|
Guru
City: Gulf Islands
Vessel Name: Soo Valley
Vessel Model: Grand Banks 36
Join Date: Jul 2019
Posts: 1,624
|
Quote:
Originally Posted by timb
I have been searching for a thread on props sizes and hull clearances .I have a single engine full keel trawler with a 24" prop and only 1.5" clearance. I know I have seen this in another post but can't find it .
also I took out my drive saver until I do a shaft alignment after the boat is splashed . debating on leaving it out. I don't know how moving it an 1.5" toward the keel and away from the rudder will effect the boat. any advice is welcome.
|
What was the reason the drive saver had to come out until you align, did that not bring the prop forward?
__________________
SteveK
You only need one working engine.
That is why I have two.
|
|
|
11-22-2020, 07:30 AM
|
#19
|
Guru
City: Oriental N.C.
Vessel Name: true heading
Vessel Model: marine trader 38 dc
Join Date: Sep 2017
Posts: 525
|
I took the drive saver out to do prop shaft alignment and try to get measurements for shaft estimates. ( it needs replacing later)It seems who ever added the drive saver just stuck it in there and didn’t modify the shaft .
I will try to get the information on wide open after I check tacks and run wide open. Not really worried about the engine it seems fine just trying to be easy on it . I couldn’t afford a new one if I lost it.
|
|
|
11-22-2020, 07:44 AM
|
#20
|
Guru
Join Date: Oct 2007
Posts: 21,326
|
"Not really worried about the engine it seems fine just trying to be easy on it . I couldn’t afford a new one if I lost it."
Regardless of the RPM reached at WOT if you are down 10% in RPM ( I use 300RPM to be safer) you should l not be overloading the engine.
__________________
|
|
|
 |
Thread Tools |
Search this Thread |
|
|
Display Modes |
Linear Mode
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
|
» Trawler Discussions |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|