1975 Starcraft Islander - Merc 165 I/O

[paparoof]

This refers to the boat in my signature.

1975 22' Starcraft Islander
1975 Mercruiser 165 hp inline 6 cyl.
Pre-alpha outdrive

I drew a line across the current prop tonight and proved it to be toast:

The boat came to me with another prop, it looks to be entirely unmolested. Previous owner said "I don't even know if this goes with this boat". Google seems to think it does. Numbers on the spare prop: 48-79574-19.

My real questions -

1) What is the recommended WOT RPM for this motor? I have manual #2, and it's been a God-send for this project so far, but it's conspicuously missing the propeller chart pages.

2) What is a realistic top speed given a total weight between 2500 and 3000 pounds? The current prop loses it at 30 mph (GPS), so that's as fast as I've gotten it to go. Tach still doesn't work yet so no idea on RPMs currently.

3) Any specific prop recommendations? The true primary usage of this boat will be just cruising and fishing (trolling) so those probably don't even apply here. Or do they? The real usage I think applies to this discussion will be pulling the kids on tubes and skis. Even the current prop feels like I'd have no problem at all getting an adult up on skis, but I haven't actually tried yet.

4) What else do I need to know?

 

[Texasmark]

Re: 1975 Starcraft Islander - Merc 165 I/O

Gotta ask the question: What was the current prop doing on the boat? Must have worked. On rpm's don't know. Why not google for your answer. The Chevy/Mercruiser engine is a torquey engine and can tolerate a lot of load. Why not try your spare and see what happens? Then make your decision. I had a friend with one in a 18' SeaRay deep V (not easily planed out) and it would do 45 mph or tow a pair of slaloms with a 4 person boat occupancy (2 on the slaloms).

Mark

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

The current prop was done from the first time I took the boat out, so I have no record of it's performance while it was good. Haven't been able to get back to the boat to pull it yet so I don't know what the numbers on it are. I do of course plan to test out the spare prop on my next run, but with nothing to compare it to except for a spun prop, I really have no frame of reference.

I did find the recommended range in a different section of the manual (page 777 in the tune-up specs): 3900-4300.

So I'll get the tach working, throw on the spare prop and see where I land at WOT and go from there.

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

Gotta ask the question: What was the current prop doing on the boat?

Sorry I guess I didn't really answer this bit -

Everything felt and worked great till I got above 30mph (GPS) then it felt like it popped outta gear. RPMs screamed and the boat slowed. I pulled the throttle all the way back to neutral, put it back into forward gear and worked fine again. Until I got back up to 30 and the symptom repeated. I pulled the boat out, went home and pulled up iboats to match the symptoms to a problem. Looked like either spun prop or bad coupler. Since I had just dismantled my homemade gantry (I had replaced the motor too - this is a complete gut and rebuild job) I was REALLY hoping it wasn't the coupler. The zillion threads I found about troubleshooting a spun prop all said to draw a line across the back of the prop, recreate the trouble, and check the line, so that's what I did. The photo above is the result - clearly the hub was spinning independently of the rest of the prop.

 

[Texasmark]

Re: 1975 Starcraft Islander - Merc 165 I/O

Sorry I guess I didn't really answer this bit -

Everything felt and worked great till I got above 30mph (GPS) then it felt like it popped outta gear. RPMs screamed and the boat slowed. I pulled the throttle all the way back to neutral, put it back into forward gear and worked fine again. Until I got back up to 30 and the symptom repeated. I pulled the boat out, went home and pulled up iboats to match the symptoms to a problem. Looked like either spun prop or bad coupler. Since I had just dismantled my homemade gantry (I had replaced the motor too - this is a complete gut and rebuild job) I was REALLY hoping it wasn't the coupler. The zillion threads I found about troubleshooting a spun prop all said to draw a line across the back of the prop, recreate the trouble, and check the line, so that's what I did. The photo above is the result - clearly the hub was spinning independently of the rest of the prop.

You did your homework buddy, now all you need to do is decide if the current prop is what you want when you get a good hub in it or do you need something else.

I mean your prop is skinned up a little, but with a new hub and the guys that do that also clean up the other asthetics and all....or it may be cheaper to buy a new prop of similar specs on here for $100 and go with that....or go all out and get yourself a SS HP and go tee-totaling-nuts....iboats has a lots of them too. Grin.

Mark

Mark

 

[elkhunter338]

Re: 1975 Starcraft Islander - Merc 165 I/O

First of all get a working tach, put your spare prop on and see how she runs.
WOT is probably around 4-4.3 k, you should be able to find it by searching the Web.

My 21' starcraft cheiftain (cuddy cabine) runs 29mph at sea level at 4100 rpms, I have the 140 hp 4 cylinder.
Its rpms range is 4-4.6k depending on the manual and the year. I like it around 4-4.3, when it revs 4.6k it sound too high.

I would expect 35mph with the 165hp, just a guess.

 

[Texasmark]

Re: 1975 Starcraft Islander - Merc 165 I/O

21' alum with a 165, shallow semi-V???? I'd say 40 would be more like it. I had a '65 18' Holiday I/O with a 110 Mercruiser and with 4 people and a day's worth of fishing gear would get 35 in a good chop.

Mark

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

Guy I bought it from said it did 35, but that was with the original waterlogged flotation foam and plywood. Now all that's gone and replaced with new fresh dry stuff so it must be considerably lighter. Maybe that means I can get 'er up a bit faster. We'll see - 30 felt pretty darn good to me, but as I get used to the boat I'm sure I'll wanna go faster.

Any guesses as to what size/pitch/whatever prop would work best for absolute top end speed on this boat?

There is a place nearby that will rent props for testing, but I only have like three weeks left in the season here....

 

[Texasmark]

Re: 1975 Starcraft Islander - Merc 165 I/O

Any guesses as to what size/pitch/whatever prop would work best for absolute top end speed on this boat?

There is a place nearby that will rent props for testing, but I only have like three weeks left in the season here....

You just hit on that too. Get busy and get some props to test...it's important to you. Keep rpms up on high end of the operating range at WOT with your NORMAL load, get your best hole shot or top end, whichever is important and if both are get a prop that does both like a 4 blade, or a ported 3 blade which I like best....get the good hole due to air across the blades and don't have a 4th blade getting in the way at WOT. Merc must think that way too with their Laser II 3 blade ported SS prop.

Usually a rental or borrowed prop means it gets back to the owner like you received it and you get your deposit back, so get in clear water and don't test it longer than necessary to make up your mind....you are testing a prop, not enjoying a Sunday afternoon on the water kind of thing will work best for you.

My current boat required 3 props to get what I wanted. The first prop that came with the boat was a 17P. How the previous owner had that boat for two years with that prop is a big question. My first time out on the water, as soon as I cleared the no wake buoys and punched it out I knew immediately that this was not enough prop....How'd I know? I planed out in about 2 seconds and immediately was at rpm redline....so obviously I had no top end.

Taking a SWAG I figured a 21 ought to be just right so I bought one of those of the same brand, same type, (minimizing variables) and it worked pretty good, but I could tell that I had some more performance in the rig and wanted all that I could get because usually I boat alone and my NORMAL load is light.

So I opted for a 24XP and got a pretty good hole...about 6 seconds, and was happy with the mph and the tach was right at the upper rpm limit....perfect or so I thought.

Well the other day I decided to improve the hole and drilled holes in the barrel, (let me tell you SS is tough stuff) thus "porting" it and now I have a nice hole shot and my top end too. I'm happy......I think! Grin.

HTH,
Mark

Good Luck

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

Well sounds like I'm on the right path then. Thanks for thinking this through with me.

I'll get the second prop installed this week and see what it feels like this weekend.

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

Update for anyone who wants to laugh at me:

I installed the spare prop (48-79574-19) and took the boat out alone yesterday. Got it up to 38mph (GPS) and decided not to push it since I still haven't addressed the tach yet. I was plenty happy with 38mph.

Then the Admiral arrived with the kids (their first ride in this boat) so I went back to the dock to pick them up. I pulled up to the non-ramp side of the dock to leave the launching space available, the family got on board, I put it in reverse and did a backwards u-turn to point the bow out to the lake. Clearly I'm used to a 14 foot boat and not a 22 foot boat because I did not realize how far I had backed up towards the concrete ramp. Put it in forward and CLUNK! CLUNK! Still took it out, but there was definite vibration that wasn't there on my first run. The kids still got their outing, but when I pulled the boat out I saw I'd broken half of one of the blades clean off. :facepalm:

Oh well, someone really wants me to go prop shopping. Just found another "gently used" one on craigslist for 35 bucks. Off I go....

 

[erwinner]

Re: 1975 Starcraft Islander - Merc 165 I/O

Hey, at least now you know the size of the spare works well with your Islander! Don't have to go looking at others.
Wish I had a 22' Islander to back into the ramp! Glad it wasn't something worse.

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

Oh this just keeps getting better and better. Bought the craigslist prop (35 bucks cheep!). Got it home, tried to install it and the blades bump the anode/rudder that lives above and just behind the prop. Digging into why, I noticed it didn't look like the prop was sitting as far into the gear casing as the other props did. Took it all apart again and found the reason. The hubs are different at the forward end.

Here's how the thrust washer fits into the forward end of the first two props:

It doesn't fit into the forward end of the craigslist prop at all:

So the prop doesn't slide far enough onto the shaft so the blades stick out a little farther than they should and bump the rudder. I was under the impression that a 15-spline Merc prop was a 15-spline Merc prop and should fit all years. Clearly I was wrong about this.

The numbers on the hub of this new prop: 48-78120-19

Found some stuff on Google that refers to this being a "black max". Don't know if that's helpful or not. Remember I'm fitting this to a 1975 pre-alpha outdrive.

Any way I can change the hub out with the hub from one of my other props or do I need to ditch this one and buy something else?

What should I be looking for as I shop yet again for another prop?

Can I get a different thrust washer made to fit this different hub style?

 

[boatnut74]

Re: 1975 Starcraft Islander - Merc 165 I/O

It almost looks like you don't need that spacer, but I havn't seen a prop where one isn't required. I looked up that prop and it says it can be used for IO and OB so I don't think that's an issue.

 

[ziggy]

Re: 1975 Starcraft Islander - Merc 165 I/O

there are two different thickness of thrust washers. looks like ya got the fat one.
i went from a old style fan blade type prop to a 19p flo torqueII prop. it needed the thinner thrust washer. even that wasn't enough to have the prop clear the old style long trim tab. a grinder cured that. there are different length trim tabs too. shorter than mine, shorter than mine and offset to aft and no trim tab, just a flat zinc that fills the hole.

don't look to pretty, but it is an mercruiser approved method of getting the prop to clear the trim tab. approved meaning that i've read it in a mercruiser service manual. seems like either 3 or 4.

fix your tach. i don't know how ya can tell if ya got what ya need if you don't know wot rpm.
fwiw. for my 165hp L6 w/ 19p flo torqueII prop. i get 4300 rpm. normal load and trimmed up higher than i normally run, but within limits. my rpm also varies as to the day. about 200 rpms. hot and humid vs. cool and dry. 4200-4400. my boat i think is lighter than yours maybe. i believe mine is supposed to come in around 2k lbs.

 

[paparoof]

Re: 1975 Starcraft Islander - Merc 165 I/O

Just stopped by the closest marine shop to my house to look for a different used prop, explained my situation and they sold me a different thrust washer - much thinner and flat on the inside flange to meet the entire hub. Looks perfect to me, but will have to hold off on judgement until I can see how it fits tonight. Fingers crossed!

Ziggy - I promise I'll get the tach wired up before my next outing (this weekend). Trust me, it's driving me crazy not knowing the RPMs. The Iboats reference says my boat weighs just under 2K pounds dry and this latest prop is also a 19p so we should should be pretty close shouldn't we? What's your top speed?

 

[ziggy]

Re: 1975 Starcraft Islander - Merc 165 I/O

The Iboats reference says my boat weighs just under 2K pounds dry and this latest prop is also a 19p so we should should be pretty close shouldn't we?

seems so. short of hull material, hull design and me being 3' shorter than you are. imho, i would think some or all of these differences could have an affect that could be different than mine. just a guess though.

What's your top speed?

i find it funny. but i can just drop the zeros from my tach reading and that's how fast i'm going (when i'm on plane). so @ 4200 i'm going 42 mph. 4300 and i'm doing 43 mph. and my fastest speed, which was this summer a few weeks ago, i achieved 4400 (she was wound up tight) gps 44 mph... other readings are gps too. i don't have a speedo.

 

[Texasmark]

Re: 1975 Starcraft Islander - Merc 165 I/O

Sir I think you are making a mountain out of a mole hill. In due respect..........

If you have a thrust washer installed correctly on the prop shaft, i.e. the washer is positioned on the prop shaft such that the taper on the washer mates with the taper on the prop shaft, then if you have any blade left over that hits your "Trim Tab" due to excessive pitch of the prop (okaye make my day...I can run the max prop pitch on my rig....grin), either get a short Merc OEM trim tab, take a file and file yours off till you have roughly a 1/4" clearance between the blades and the tab, or just take the "thing" off....if your steering can tolerate it. I am currently running a "trimmed" trim tab and it works beautifully....have pics posted 25 July '11 under my "handle herein".

Mark