Ski-Tow performance question

Designs for inboard or outboard power

Moderator: BruceDow

Mark-NJ
Posts: 252
Joined: Sat Feb 06, 2016 5:55 pm
Location: New Jersey

Ski-Tow performance question

Postby Mark-NJ » Sun Sep 03, 2017 1:22 pm

Well, the boat got launched!

Everything went well, but "Grandma's engine" barely made it through the maiden voyage. A bad bearing in the powerhead revealed itself about halfway between the municipal launch ramp and our cabin on the other side of the lake. And, honestly, 15HP is too small for this boat anyway. With only me in the boat, the 15HP pushed me about 15MPH (according to my iPhone), journal bearing screaming as I went.

With the engine declared dead, I went on Craigslist and got lucky....found a '69 Evinrude 25HP for cheap, and it saved the vacation!

Still, 25HP seemed a wee bit small. With just me, the Ski-Tow zipped along at 23MPH.....with 4 adults, 18MPH. Not bad, but I wouldn't mind a wee bit "more".

But I'm re-thinking this: I'm now noticing that the entire time we ran the boat, the engine's tilt pin was in the first hole, so the engine was hanging on the transom NOT properly trimmed. To make the engine aligned perpendicular with the keel, the pin should be in the 3rd hole, to compensate for the transom's 13 degree slope.

So here's my question: how much performance did I lose with the engine improperly trimmed? I've been thinking I need a bigger engine, but I'm now wondering if proper trimming would give me a little better performance.

Thoughts?

PeterG
Posts: 351
Joined: Mon Dec 03, 2007 7:08 am
Location: Connecticut

Re: Ski-Tow performance question

Postby PeterG » Sun Sep 03, 2017 1:44 pm

Probably robbed half the speed it could have had? Proper trim makes a big difference. With the boat level in the water, the engine should be upright, vertical. Basically with the cavitation plate should be horizontal and at or slightly below the bottom edge of the transom (probably just triggered a hot debate about cavitation plate height). With your tilt pin in the first hole, the motor was angled such that it would tend to lift the stern and push down the bow, plowing too much water. Try trimming your motor like I suggest and see if that helps. 25hp seems a bit underpowered but I have nothing to base that on, it all depends on what you want for performance. Maybe a 35 or 40hp motor would be more to your liking? More than that and you might need to stiffen/reinforce your transom.
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

User avatar
Bill Edmundson
Posts: 10444
Joined: Wed Nov 23, 2005 6:45 am
Location: Birmingham, AL, USA
Contact:

Re: Ski-Tow performance question

Postby Bill Edmundson » Sun Sep 03, 2017 3:40 pm

Mark

I found on the Bartender, with power tilt, when I hit the sweet spot the rpm really went up, and speed.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Mark-NJ
Posts: 252
Joined: Sat Feb 06, 2016 5:55 pm
Location: New Jersey

Re: Ski-Tow performance question

Postby Mark-NJ » Sun Sep 03, 2017 4:09 pm

I'm buying a "project" 40HP tomorrow....and based on the description, it should be pretty easy to get it running, so the "do I need a bigger engine?" question might be moot.

Still, the 25HP was pretty decent...I just wonder how much better it could have been, if trimmed right.

User avatar
vupilot
Posts: 3235
Joined: Wed Apr 15, 2009 7:36 am
Location: Indy
Contact:

Re: Ski-Tow performance question

Postby vupilot » Sun Sep 03, 2017 7:12 pm

In my experience you probably only lost 2-3 mph. The speeds you were seeing with the old 25hp are pretty decent even with the poor position so I doubt you will gain but a couple mph. Keep in mind 25 hp back prior to about 1983 was rated at the crankshaft and 25hp since then is rated at the prop. So a early 80's and newer motors will be a couple horsepower more than a pre-1980's one.

30hp is probably the sweet spot if going with modern power but and old 40hp if you can find a good reliable one will be all the power you need and keep the weight reasonable. New 40 hp motors just weight too much and are HUGE looking.

Mark-NJ
Posts: 252
Joined: Sat Feb 06, 2016 5:55 pm
Location: New Jersey

Re: Ski-Tow performance question

Postby Mark-NJ » Mon Sep 04, 2017 3:18 am

good info...thanks!

I'm on my way to upstate NY to get a '71 Evinrude 40HP. Needs work, but nothing major (or so I"m told!).

EDIT: So The roadtrip was worth it.....I now have a (non-working) 40HP in the garage. I've restored engines before, so this one should be no problem. Compression is 100 in each cylinder, but the lower skeg seal leaks and one cylinder isn't getting a spark. And it's filthy. But these are all easily fixed.

This thing is heavy, however......155 pounds. I have no qualms about my transom supporting it and pushing the boat through the water, but I'm not sure I'm comfortable trailering the boat with the engine on it. I can foresee a good pothole causing this thing to tear the transom out of the boat. After all, weight & forward thrust (in the water) is not the same as shock, moment, and inertia. I think this beast will ride in the back of the truck.

That said, my transom is double-thick sapele (1-1/2" total thickness) that's completely screwed & epoxied together. The knee is oversized 2-1/4" thick (a sandwich of three pieces of 3/4" marine ply) and tall, bolted and epoxied to the transom & keel. The keel is double thick sapele (1-1/2" total thickness, 6" wide), screwed & epoxied to the transom. The battens (6 of them) are likewise screwed & epoxied Each batten is doubled up at the transom, to compensate for any weakness crated by the limber cuts. The sapele carlings and deck boards are screwed & epoxied to blocks on the transom which are, themselves, screwed & epoxied to the transom. The meranti plywood bottom (3/8") & sides (1/4") are screwed & epoxied to the transom edges. There's no "engine box", as I built my hull as an open "utility" style, so I lack that strength.

So am I worried needlessly about the weight of this 40HP? The notion of the engine & transom tearing out on Rt95 is more than I can fathom! :)

User avatar
vupilot
Posts: 3235
Joined: Wed Apr 15, 2009 7:36 am
Location: Indy
Contact:

Re: Ski-Tow performance question

Postby vupilot » Mon Sep 04, 2017 6:43 pm

Thats a little on the heavy side but not crazy. My Zip has made the 850 miles round trip to the Gathering 4 or 5 times with a Mercury 402 40hp on the back and never had any problems. Todd Broadlick had your same motor on his Zip and trailered fine too. I'm sure your transom is just like the Zips in design. You dont want to be lifting a 40 hp motor very often even with two people. I encourage the use of a transom saver and I'm sure youll be fine.

Hercdrvr
Posts: 366
Joined: Mon May 11, 2015 12:52 pm
Location: McKinney TX

Re: Ski-Tow performance question

Postby Hercdrvr » Tue Sep 05, 2017 4:02 am

Your ski tow transom is the same as my Malahini. You'll be fine with a 40hp.
Matt B
Attachments
IMG_2773.JPG

Mark-NJ
Posts: 252
Joined: Sat Feb 06, 2016 5:55 pm
Location: New Jersey

Re: Ski-Tow performance question

Postby Mark-NJ » Tue Sep 05, 2017 4:09 am

My transom isn't like these designs....I don't have a motor well. I built an open-back utility (like an old Lyman), so while my transom is still strong, it doesn't have all that reinforcement.

Still, I guess with a transoms saver, that would surely help. And you're right: this is NOT an engine that I want to be taking on & off repeatedly. I think the fact that it has a lifting ring on the top of the powerhead is the hint that this is not meant to be man-handled!


Return to “Power Boats”

Who is online

Users browsing this forum: katees2 and 10 guests