Print Page | Close Window

Mac 360 Porpoising

Printed From: The Fishing Website
Category: General Forums
Forum Name: The Boat Shed
Forum Description: Discuss all things boating.
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=131170
Printed Date: 29 Mar 2024 at 6:10am


Topic: Mac 360 Porpoising
Posted By: changnxf
Subject: Mac 360 Porpoising
Date Posted: 28 May 2019 at 9:50pm
Hi, in need of advice please.
Current setup:
Mac 360 Year 2000 (low transom)
Panther 35 electric tilt
Tohatsu 30HP electric start, tiller control, short shaft

Problem: Boat is porpoising in flat water, motor appears low in water.  Played with trim, not much difference.

Motor is mounted in accordance with Tohatsu manual with anti-cavitation plate at 30mm below bottom of hull.  Other posts suggest that the plate should be level with bottom?

Is motor too low?  Would Permatrim or equal help?

Thanks.



Replies:
Posted By: Bigfishbob
Date Posted: 29 May 2019 at 8:47am
Yeah motor is way too low, the cav plate should be dry or with droplets running over it when you are at WOT. If the engine is too low the boat will porpoise more. porpoising is also caused by being trimmed up too far.

-------------
www.waikatosportfishing.co.nz


Posted By: Bounty Hunter
Date Posted: 29 May 2019 at 8:51am
tohatsu manual suggests antiventilation plate to be 30mm BELOW the keel?

did you read that right?


-------------
No disintegrations!


Posted By: Don18025
Date Posted: 29 May 2019 at 9:41am
You maybe over powered?
Does it porpise at 2/3rd WOT? 
If NO, then I think you have to much power on a small boat.


Posted By: OneWayTraffic
Date Posted: 29 May 2019 at 11:22am
30mm below keel is what you might use for a workboat or slow speeds. Typical is level with keel then take it from there. Some even go 30mm above keel or even more in racing craft. 

Aside from that your boat will not be in good trim with you, fuel tank, motor all in the back. That's just simple Physics. 

If with a mate, put them up front. Otherwise consider tiller extension or something to move weight forward a bit. Even a few jugs worth of sand in the front. Everything you can manage.

After all that you could consider a hydrofoil, or even SLT trim tabs but much better to fix the weight distribution and motor height first. 






Posted By: Steps
Date Posted: 29 May 2019 at 11:46am
Different makes design venation plates at different heights
 eg Merc is low.
 Yammy/ johnny,Evinrude, the ventilationplate bottom touches the water with a few droplets running down the top surface.

Also the distance back from the bottom of the hull makes a difference as to setting and measuring a straight line off the hull surface.. further back higher in relation the the straight line.

One can set up the height exactly right, but in most cases needs to drop about 1/2" to prevent venation in tight turns and chop.
Issue here is  most adjustments are in 1" increments, so one must decide on flat water or chop conditions.. there the end use of the boat comes in.

Tohatsu  seem more like a merc set up where the smaller upper plate bottom surface is just touching or slightly out of the water and the plate under water at fast cruise , best trim on flat water, normal boat loading.

We have found on mercs, and ASSUME tohatsu to be the same. The 1st setting set the top upper small plate slightly or 1/2" above the straight line from the bottom of the hull.

Then sea trial, normal load flat water fast cruise speed etc.
Do a few tight turns and see if ventilates .. it is normal to trim down (if have trim) before a tight turn.
 Experiment with your normal turns also, is there ventilation, or enough ventilation to be of concern in how YOU load and operate your boat.
For best general  fuel efficency, boat performance etc you will have a little ventation in turns

You will either be happy with that height setting or decide to put slightly higher or lower.

The effect of having an engine too low is huge on the fuel efficiency and cruise speeds etc...even an 1" or so on a small 4.5m boat is like adding an extra heavy person..

The next thing once the height is correct is getting the pitch and diameter of the prop right.
 Pitch:
You will be aiming middle of the range or slightly below the max rpm range with normal load at best trim on flat water.
 Diameter:
A slip factor approx 5% at WoT and around 12% at cruise speed

As to stainless or alloy.. going stainless in the case is a waste of money as  mostly cruise, light boat, small prop loading etc, and most like getting close to reefs, shallows.

If after all that and the bow still rides heigh, a prop, usually a 4 blade with more rake will fix that.
 With that 30hp on the back there is no way you should ever fit a perma foil to the motor

 As to power, you are well powered, from memory that about the top of the spec for that boat...

In theory would be 
about 450 kg gross weight (??) on the water, 
30hp would give around 32/35mph WoT @ around 5500 rpms 
with about a 14" inch pitch prop
 And 3800/ 3900 rpm a cruise around 23/24 mph

These are very much estimated ball park numbers





Posted By: MacSkipper
Date Posted: 29 May 2019 at 12:28pm
I had a Mac360 with Panther unit - remember effectively the Panther works as a pod moving outboard out 4 inches from transom - this means (as I recall) motor has to be higher because water curves up after leaving transom as it is in water - plenty about this online or post a photo from side with motor down.

-------------
Good fishing trip nothing breaks, great trip catch fish.


Posted By: Bounty Hunter
Date Posted: 29 May 2019 at 1:51pm
Originally posted by MacSkipper MacSkipper wrote:

I had a Mac360 with Panther unit - remember effectively the Panther works as a pod moving outboard out 4 inches from transom - this means (as I recall) motor has to be higher because water curves up after leaving transom as it is in water - plenty about this online or post a photo from side with motor down.

yep - this will be very relevant 


-------------
No disintegrations!


Posted By: Steps
Date Posted: 29 May 2019 at 5:18pm
Originally posted by MacSkipper MacSkipper wrote:

I had a Mac360 with Panther unit - remember effectively the Panther works as a pod moving outboard out 4 inches from transom - this means (as I recall) motor has to be higher because water curves up after leaving transom as it is in water - plenty about this online or post a photo from side with motor down.

I mentioned the distance from the rear of the hull.
It curves up in an elliptical curve... hence why the rule of thumb, X inches up for Y inches back is at best a guide line . So many variables  weight at stern , shape at the rear of the hull etc


Posted By: changnxf
Date Posted: 29 May 2019 at 11:47pm
Thanks everyone for the feedback.  There's a lot to absorb.  I'll try to attach a photo of the relative height of anti-cavitation plate to boat hull.  I get that the further back the motor is, the higher up it CAN be.  But how high?  I reckon that because of the mounting set up on the Mac transom, I'll have only one more shot at it, probably got about 30mm.

Hmm, sorry but I couldn't get it the right way upErmm
Bottom of ply is top of the anti-cav plate.  There is about 30mm height difference.  The setback is about 200mm.
Thanks.


Posted By: MATTOO
Date Posted: 30 May 2019 at 8:08am
Is the trim default modifiable.

Most common fault is boat balance and as mentioned the transom angle to motor.

Some engines have a control setting in the tilt that is default.
Many shops set that and even when sea trailing still set the boat so it porpoises.
It's believed to be safer by keeping the bow up.
This negates the ride quality.

If the owner or boat user understands this and the trim has a readjustment around the trim default this is the easiest way to modify the ride.
It then means that in certain sea conditions you could be plowing or having to much bow down.
This is a risk, but then you just change the trim up and the ride is safer.

If you do not have a default setting change in your system then the alternative is to use packers on the outboard to transom mount.

This area I have described is your first port of call before other areas mentioned.

Remember all boats will ride better with a well balanced boat.
Use the weight you carry to either fixed or loose gear or passengers to your advantage.
Balancing a boat is sensible and critical for all boat sizes.

-------------
Just cruising in my now sweetas pimped out Southern 755 HT0!


Posted By: MacSkipper
Date Posted: 30 May 2019 at 8:33am
Mattoo is incorrect this is a basic after market tilt trim doesn't have a set point or anything like that.
 
Looks too low in picture 30mm may not be enough - I would take boat out on a calm day 2 people in boat (one to drive) and normal load (if normally solo take some gear out maybe?) -  get on plane at cruise as this is where I try to set up -trim till skid plate parallel with water if angled in or out will kick up spray/motor note will change,  Once at that point (with you at back of boat already so trim not changed) look at skid plate should be (steps quote) droplets of water plate just in water - will be to low (submerged) I expect and how deep is how high you need to lift motor.  You will be surprised how much higher it has to go I thought mine was to high after shop fitted but was spot on (don't have boat anymore so no photo sorry).
 
Good luck!


-------------
Good fishing trip nothing breaks, great trip catch fish.


Posted By: Steps
Date Posted: 30 May 2019 at 8:51am
Take into consideration Mattoo comments above re trim.. fixed trim
 Trim has a huge effect...
With adjustable one starts all the way down, this enables the bow to lift over the bow wave, get on the plaine easy.
 Once over the bow wave, one then trims up )the engine prop moves out from the stern...
One , on flat water trims up till just before proposing, and/ or ventilation.
As the speed increases, the curve of the water surface between hull bottom and prop changes and the depth at the prop changes...There fore one can increase the trim.

Going into another boats wake at cruise, with a nicely powered boat, or going into chop, one simply trims down , rather than pulls back on the throttle (within reason)

So as you will see the fixed trim setting  will be a compromise. Done and /or by adjustments on the engine mounting / wedges between engine and  stern.
 A river/ lake boat will tend to have a little more trim than a salt water boat..

 I'll try to attach a photo of the relative height of anti-cavitation plate to boat hull.
 In practice even on larger boats it is often hard to take pics.. the easiest way , practically, is video mode and wave around both sides the general area.. you are sure to get good shots 1st time. 

Hmm, sorry but I couldn't get it the right way upErmm

 thats ok , downloaded it, turned it and zoomed in.
 Edit your pics before uploading.
1/ Flip the pic if required
2/ crop all the stuff not needed around the outside. (in this case pretty good)

Bottom of ply is top of the anti-cav plate.  There is about 30mm height difference.  The setback is about 200mm.
Thanks.
 A close look may indicate your straight edge maybe not quite in line with the hull... 1mm make quite a difference by the time you measure the motor

 The 30mm as you gather is an indicator... the actual critical depth is how far the prop is under the water...also of side note, a higher rake prop can be lifted higher.
 To me looks like engine has been set to initial ball park, and not sea trialed and then set correctly.
Another thing, the ventilation plate (trim) is set parallel or near parallel to the hull...
Here we go back to Mattoos comments, 

Some engines have a control setting in the tilt that is default.
Many shops set that and even when sea trailing still set the boat so it porpoises.




Posted By: Steps
Date Posted: 30 May 2019 at 10:06am
Mac I said before with Tohasu and ASSUMED same as a merc.
 And yes althu a merc is 30 below, it is also very common to have them much higher on race boat type applications.. 

So yes, would not be surprised if the tohasu could be lifted higher without ventilation.

Mac had 2 of these boats and both where very well setup..including bar crossings.

I'll have only one more shot at it, probably got about 30mm.
 Drop it up to that then sea trial..
it is easier to have too high , have venation and drop, rather than slowly going up.




Posted By: MATTOO
Date Posted: 30 May 2019 at 7:43pm
In respect of porpoising and thank you Steps.
The balance of the boat re trim is always the number one culprit.

You can still have a dog of a boat design though.which can be hell to solve.

Engine height is an excaerbator of a number of issues.
Fuel consumption, handling and possibly dangerous engine and hull stresses .

The success of solving your issue comes with steps.

Step one is as I have pointed out.

I accept that macskipper may have solved his problem as he described.
More than possible.

But may be he thought it was solved.
Through experience I offer to give a greater volume of knowledge to clarify a picture.
No offence meant here macskipper.

I'm resting on my fifty years of experience and learning which is far from absolute.
I live to learn or be corrected by experience of others or those more learned than I.

That is the value of forums.
That is the value of learning, doing and conversing.

However, it is just my opinion.

-------------
Just cruising in my now sweetas pimped out Southern 755 HT0!


Posted By: Bigfishbob
Date Posted: 30 May 2019 at 9:43pm
I'l say it again, mount the engine as high as it will go. If it cavitates when you turn them drop it down a hole. the further away from the vee the axis of the prop is, the more it will try and lift the bow as you travel. Get the cav plate riding as high as you can then go from there, a permatrim MAYBE your next step, or even better a higher lift prop (low rake 3 blade or 4 blades). to lift the transom more. When you lift the transom then the bow is less likely to bounce. Keep it simple.

-------------
www.waikatosportfishing.co.nz


Posted By: changnxf
Date Posted: 30 May 2019 at 10:24pm
Thanks for all the feedback.  I am going to have to do some proper 'sea trials', weight at the bow, checking out the full extent of the electric tilt (trimmed down) etc.  Had a look at some pics (boat is not at home) and the top bracket of the motor is pretty much in line with the top of the transom, which is where it would have been without the Panther.


And it looks like I actually have about 60mm of height to play with but as I mentioned previously, I reckon I have only one more shot at it because these small motors do not have multiple holes for height adjustment and I have to drill!
I'll let you all know how I get on.   Thanks again.


Posted By: Steps
Date Posted: 31 May 2019 at 10:11am
and the top bracket of the motor is pretty much in line with the top of the transom, which is where it would have been without the Panther.

I can only comment on a V4 and V6 115/150 hp jonnys
 The min distance of the top hole from the top of the transom is 3/4"
 I THINK Yamaha and mercs of similar size are also.

 If the engine .. as Mac.. alludes to has been mounted without the panter and the panter being added later then it is very likely .. because of the extra distance between prop and keel, the engine by default could have the mount holes drilled incorrectly for your application.

Before you start drilling etc.. ring Mac boats.. have a chat
 They are very knowledgeable and helpful.


Posted By: harryh
Date Posted: 01 Jun 2019 at 2:25pm
You will find that for every foot the engine is stepped back from the transom you go up an inch from the bottom of the boat. It looks to me that you need the cav plate about 30mm above the bottom of the boat. The panther brackets add weight to the rear of the boat as well. Not sure how old the hull is but have you run a straight edge along both sides of a hull. With the plastic boats sometimes they can change shape (hence the reason they are on skid trailers rather than rollers) if the hull has a high or low point in it that can cause porpoising to


Posted By: Slapper 61
Date Posted: 16 Sep 2019 at 10:06pm
Hi. I realise this is an older post. Did the problem get fixed and what was the fix?

I have a Mac 360 with a 30 merc. I haven’t had it that long and it porpoises at cruise speed but if you get closer to WOT then it’s fine, but your doing about 30 knots in a 12 ft dinghy so not the best in all conditions.

I have the fuel tank up the front. And it has a hydrofoil which I fitted to help with towing biscuits. Also it gets a lot of nose rise when trying to get on the plane when by yourself which was another reason for the hydrofoil, but I think that’s pretty typical for these.

Was the engine height or trim the main issue?

I suspect it maybe worth trimming the engine down perhaps 1 hole at a time. It’s manual tilt so it needs to sit at best point for all ranges. Or would adjusting height likely make more difference?

Thanks in advance if anyone has found and fixed this on a Mac.


Posted By: MacSkipper
Date Posted: 17 Sep 2019 at 4:41am
I had same motor and boat couldn't get it right at all speeds ended up fitting a power trim and adjusting at different speeds. This hull is sensitive to trim, manual trim adjust till you find sweet spot for speed conditions and load on day. With trim in fast level holeshot but hairy at speed but trim out as speed came up and all good. Powered trim is expensive to retrofit.

-------------
Good fishing trip nothing breaks, great trip catch fish.


Posted By: Steps
Date Posted: 17 Sep 2019 at 10:50am
Read thru older posts..
 Check engine is correct height.. WITHOUT the permatrim total waste time doing anything if that is wrong.
 Once correct height.
 Then look at correct prop.. if still porpoising when correctly trimmed at each speed...
 Different speeds require different trim.. faster you go the more trim.
 If still have issues, then look at a 4 blade prop with more rake... blades lean back more. This levers the stern up and bow down.

 Get and know height is right 1st.


Posted By: changnxf
Date Posted: 19 Sep 2019 at 7:43am
Thanks Slapper 61
Prior to the electric start Tohatsu 30, I also had a Merc 30 (same short shaft motor) with the Hydrofoil.  Also with tank at front - under front bench seat - had no issues with porpoising at all.  Fixed trim was set to 2nd to last hole.  Admittedly hardly ever on my own in it, always at least someone on front seat.  
Unfortunately I haven't had a chance to resolve the current porpoising problem, but I am definitely going to relocate the motor up a few centimeters.
I will post any results.


Posted By: Steps
Date Posted: 19 Sep 2019 at 5:09pm
If motor correct height, and porpoising, then reduction in trim is required if the balance loading of the boat is the same.


Posted By: Snappa Geoff
Date Posted: 20 Sep 2019 at 4:56am
Just a thought Changnxf, have you taken measurements on the later model 360 with high transom with same outboard on as yours? Replicate height of outboard from where shaft sits at bottom of hull with a mark on shaft then line up mark with the bottom of your hull and that will give you your mounting height. I have later model 360. Used both long shaft merc 30 and have Yammy 30 on now, never had any problems, even beach launching on my own. Beer


Posted By: changnxf
Date Posted: 02 Oct 2019 at 10:20pm
Thanks Gents
I'll be able to get on to it soon.  All Advice is appreciated.


Posted By: changnxf
Date Posted: 08 Nov 2019 at 11:26pm
Gents 
Thanks for all the advice.  An update:
I raised the Panther tilt so that it was in line with the top of the transom, lifted by about 60mm, and the anti-cavitation plate is now above the bottom of the hull.  Trimmed right down, lot less bow up on getting up to plane, and with 1 light person and a bucket of sand up front, was able to find a comfortable speed without porpoising.  With two people up front, there was no sign of porpoising at all.  Outboard is now at more acceptable height and not looking like being immersed in the water.  Lot less spray from the motor as well.
Not planning any more adjustments.  Thanks again to all.


Posted By: MacSkipper
Date Posted: 09 Nov 2019 at 6:10am
Originally posted by changnxf changnxf wrote:

Gents 
Thanks for all the advice.  An update:
I raised the Panther tilt so that it was in line with the top of the transom, lifted by about 60mm, and the anti-cavitation plate is now above the bottom of the hull.  Trimmed right down, lot less bow up on getting up to plane, and with 1 light person and a bucket of sand up front, was able to find a comfortable speed without porpoising.  With two people up front, there was no sign of porpoising at all.  Outboard is now at more acceptable height and not looking like being immersed in the water.  Lot less spray from the motor as well.
Not planning any more adjustments.  Thanks again to all.
Thanks for posting outcome great to hear!

-------------
Good fishing trip nothing breaks, great trip catch fish.


Posted By: changnxf
Date Posted: 22 Mar 2020 at 10:49pm
Gents
Further update on the porpoising problem.  Have fitted a LZ Hydrofoil on the (not so now) new Tohatsu just like the old Merc 30 and we are getting up and planing very easily, no bow up issues and no more porpoising. Thanks for all your comments and advice.
Regards



Print Page | Close Window