1978 Johnson 70 on Glasply Deep V, What to do? Tested...

[billy4hp]

Just fishing for some suggestions on what I should or should not do...

I have a 1978 Johnson 70hp 25" shaft mounted on a 1972 Glasply 16' deep V runabout. For those of you not familar it is a very deep V boat with full length spray rails for ocean running, it is designed to have alot of hull in the water to cut through the expected rough water. The dashboard / controls are forward in the hull, much further than a comparable Bayliner or similar runabout. The boat weighs around 1000lbs dry without engine or gear. I normally carry two 6 gallon tanks in the rear, marine group 24 battery, I have a 5 hp Nissan kicker mounted on the transom as well... I carry the normal ice chests, anchor, ropes and gear the Coast Guard up here expects us too. Usually for passengers I have myself and atleast one of my two boys (only 6 years old on my oldest).

The boat currently has what I believe to be the original 13 1/4 X 17 prop. In it's current position the caviation plate is flush with the bottom of the hull V. The engine has no power trim although I have played around with the trim bar and have found my happy medium for speed and control. The boat has Bennett trim tabs to push the bow down in rough chop for a smoother ride and to control listing with a heavy load (or passenger).

My engine is rated for a 5500 rpm max @ WOT. I can get nearly that (54XX and change on a smooth surface) and run around 36 to 38mph... Running with the current through Deception Pass I do get the joy of travelling 45mph ...

My question is I have read where a boat will run better with the caviation plate above the bottom of the hull surface? Also I am sure there have been plenty of achievements in prop design since 1978.... So how much am I leaving on the table with my current setup? Will a newer 13 1/4 X 17 prop give me a noticable increase in performance? I have been planning on buying a new prop just as I believe I am running on borrowed time with the original 1978 prop as far the the rubber hub goes??? Since we can get a few miles offshore it's advisable up here to have a replacement prop anyways just for a comfort factor...

Too be honest most of my running in the ocean is well below my WOT max as water conditions can get kinda nasty in a hurry or if a fleet of 36 foot plus yachts are running around you have instant 3 and 4 foot seas. So for the information I usually run around 26 mph @ 3600 to 4100 rpms (saves on fuel too)...

Thanks for your time,

Billy

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Any thoughts???

 

[junior1113]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

first thing i always look at is set up to achieve max rpms with current prop. where is cavitation plate in relation to bottom of boat? can you post a pic? best way to measure is put a straight edge from bottom of hull to motor.

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Unfortunately it's raining today so I won't be able to get any pics up but I did take a look with the engine down and it's about 1/4 below the hull using a straight edge. My engine is mounted in the top most hole so I have four holes of adjustment to play with to move the engine up...

Thanks for the help,

 

[countvlad]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Unfortunately it's raining today so I won't be able to get any pics up but I did take a look with the engine down and it's about 1/4 below the hull using a straight edge. My engine is mounted in the top most hole so I have four holes of adjustment to play with to move the engine up...

Thanks for the help,

acording to you, you are pretty much at the highest WOT possible RPMS... dont change the prop... the prop that you have now is fine... after you post the pics i think you could proably raise the motor a hole and go from there... i dont think theres much that you can do, you got all the juice out of the motor ... can you trim the motor? good luck

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

acording to you, you are pretty much at the highest WOT possible RPMS... dont change the prop... the prop that you have now is fine... after you post the pics i think you could proably raise the motor a hole and go from there... i dont think theres much that you can do, you got all the juice out of the motor ... can you trim the motor? good luck

I can trim the motor by moving the tilt bar from hole to hole to change the angle and that's about it (it's a manual tilt model). I do have trim tabs to adjust for listing and rough water conditions (forces the bow down)...

Since I know I'm about maxed out for RPM I was hoping to get some info on what a newer 13 1/4 X 17 prop will do (since my current prop is the original looking 1978 vintage)... I am considering a Solas Amita or Turning Point Hustler II.... Any suggestions?

Thanks again...

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Here is a picture of the transom of my boat with the engine in it's current position....

With a straight edge the cavitation plate is about 1/4" below the bottom of my hull...

Lemme know what you think

 

[countvlad]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

from the picture seems like the cav plate is even with the bottom... try raising the motor a hole and go from there

if you plan to change the prop and go with a higher pitch im not sure how big of a impact is going to be.. you might gain few miles/h

 

[hwsiii]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Billy, in my opinion you need to buy a 4 bladed prop like the solas amita you are looking at. The advantages for your particular uses are many. It will have a lot more holding power when you are in head seas or in following sea, meaning it will require less throttle jockeying to keep the boat form surging and bogging. It will let you plane slower in confused seas when the waves are coming from multiple directions and you need to be going as slow as possible but yet need to have control by being on plane. It will also give you better midrange planing speeds than what you have now.


H

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Billy, in my opinion you need to buy a 4 bladed prop like the solas amita you are looking at. The advantages for your particular uses are many. It will have a lot more holding power when you are in head seas or in following sea, meaning it will require less throttle jockeying to keep the boat form surging and bogging. It will let you plane slower in confused seas when the waves are coming from multiple directions and you need to be going as slow as possible but yet need to have control by being on plane. It will also give you better midrange planing speeds than what you have now.


H

hwsii,

Given my WOT rpm with my current 13 1/4 x 17 (nearly the recommended limit of 5500) would you suggest a lower pitch with the four blade? (I remember reading with a four blade you generally go down in pitch)....

After checking Iboats store my only options are either staying at the 17 pitch and going to a 12 3/4 dia or going to a 15 pitch at 13 dia if I want to go with the Amita prop (which I do, it's onsale)...

Thanks for the info,

 

[hwsiii]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Billy, get the 17" pitch and then if there is too much drop in RPM we can work it out by raising the engine. What is the gear ratio for your motor.


H

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Billy, get the 17" pitch and then if there is too much drop in RPM we can work it out by raising the engine. What is the gear ratio for your motor.


H

hwsiii,

Per my OEM manual my gear ratio is:

Long Shaft (25") propeller gear ratio 12:29

Thanks for the help, I was all ready to order a 15 pitch per the Iboats propeller 101 guide...

 

[hwsiii]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

I am glad you didn't, you would be turning wayy to many RPM.



H

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

hwsiii,

Ordered the 12 3/4 x 17 4 blade from iboats... free shipping and 40 bucks cheaper than I could buy it locally....


When it arrives should I install it and move the motor up a bolt hole straightaway? My boating time is kinda limited so if you guys figure I would benefit from it anyways I rather make the changes all at once rather than do one thing at a time for the sake of testing...

I appreciate all the info and will share the results as soon as the new prop arrives...

 

[hwsiii]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

My only concern with raising your motor up is that in rough seas the boat at times tends to have the bow raised and then the stern will be raised after that as the wave passes. This is not like running in quiet water where you have very little motion from seas. The higher the motor is out of the water the more chance there is for ventilation and that can mean loss of power at a critical time, that is why I wanted to wait. But if you want to go ahead and move it up one notch.


H

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

My only concern with raising your motor up is that in rough seas the boat at times tends to have the bow raised and then the stern will be raised after that as the wave passes. This is not like running in quiet water where you have very little motion from seas. The higher the motor is out of the water the more chance there is for ventilation and that can mean loss of power at a critical time, that is why I wanted to wait. But if you want to go ahead and move it up one notch.


H

I agree with what you are say and have in fact experienced it first hand (ventilation while working rough conditions)... So with that being said I will install the prop first and see how it goes, it should be interesting to see how the reduction in diameter (13 1/4 vs 12 3/4) with the additional blade affects performance and WOT rpm...

Most importantly this is the first week of crabbing so I gotta go get some pots set....

 

[hwsiii]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

I hope you catch a lot of those dungeness crabs, they are delicious. I have no doubt you will be very happy with the extra low end holeshot and acceleration provided by the extra blade. In my opinion when you are running in areas that have sand bars and strongggg currents from rivers you need all the low end power you can have. I know in Washington state they lose some crabbers every year to the opposing river currents and onshore waves that can create very large confused seas at times.


H

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Just wanted to say thanks to Iboats for the quick delivery on my new prop and the great price (almost 1/2 of what I would have paid for it locally, same prop)...

I should be able to get out on the water Thursday night and will post up the results...

FYI after removing my old prop, my original thoughts held true, rubber in the hub was severely cracked and I found several stress fractures around the prop.... Glad I decided to be proactive instead of reactive....

 

[hwsiii]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Billy, that could have been real dangerous under the wrong circumstances where you boat at. PROACTIVE


H

 

[billy4hp]

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

Re: 1978 Johnson 70 on Glasply Deep V, What to do?

First off thanks for all the replies, I would have been way off base picking a prop myself...

Okay here are the results:

I purchased (from Iboats of course) a Solas Amita 4 alum prop. 12 3/4 x 17 pitch.

Before the boat performed well, it had a 3 blade original 13 1/4 x 17 pitch prop. I would WOT 54xx RPM with a usual load a run anywhere from 34 -37 mph. With a very light load in salt water I would occassionally see 40 mph running with a strong current.

Now the boat does everything it did before but better. We went out this evening on our local freshwater lake in the rain... The lake is basically sea level as far as altitude goes. I never touched my trim tabs, even after constantly crossing my own wake trying to verify what I thought I was feeling. Before the boat would do the typical bow rise when going on plane (getting on plane was never a problem before) now it simply accelerates and is on plane in about 2 - 3 seconds max. We had our average load and I figure the boat, loaded, weighs 1800lbs with boat, engine, gear and people. On my GPS we saw 36.7 mph so when we go into the salt water we will see around 38 mph if prior experience holds true, with a average current we will run over 40 mph easy when going with the flow, running through Deception Pass with a big current will be it's typical white knuckle ride.... http://en.wikipedia.org/wiki/index.html?curid=8259

The biggest plus is how this boat now handles, before I constantly was trying to find the balance of bow lift by using my Bennett Tabs to push the bow down. The sweet spot was fairly narrow. Even with the engine tilted in as far as possible it always held the bow up which in my application and sea conditions isn't a good thing. Now the bow is down and for WOT RPM I saw a constant 5530 RPM and it only pulls down to 5400 RPM in a hard turn. It never had any prop blow out. Cruising along I am easily pulling 500 RPM less than I did before and running 3 or 4 MPH faster with the new prop, it has that much more stern lift...

So I am a happy camper thanks again to everyone...